The thing that separates Katherine Heigl from being just another Hollywood blonde is her likability factor. She tries, but not too hard. She’s pretty, but not too pretty. She makes an effort, but not too much of one. She’s got good taste – the vermilion outfit she wore for the scarlet carpet at the Oscars made her stand out, not blend in. But it didn’t scream “look at me, I’m it,” even though the current heads-of-studio thinking is that she is. She was neither winning nor presenting an award, yet it was Heigl in her off-the-shoulder Escada – cinched in at the waist and flowing with its train, a classic, old-fashioned Hollywood look – that made the best-dressed lists.

She always seems to smile freshly. Plenty to be amused about. Two years ago it was said that she was earning less than her Grey’s Anatomy co-stars. For Knocked Up, last year’s summer smash directed by the new cool of comedy Judd Apatow, she received $300,000 for playing a girl who becomes pregnant after a one-night stand with a pot-bellied slacker. For this spring’s 27 Dresses, which has already been No 2 in the US, she is reported to have been paid $6m, and she is being called the new Julia Roberts. Or Meg Ryan or Sandra Bullock. Meaning there was a gap in the market that needed to be filled with a new leading lady of romcom for a new generation. She’s a little sniffy and a little pressured in equal parts about the notion of being called the new anybody.

“I can’t help but feel that’s a little insulting to Julia Roberts,” she muses. “There’s not another woman who I look at and think, ‘That’s it, that’s whose career I want to have.’ I’m just me and trying to do the best I can in every decision in any given moment,” she says with a curious mix of poise and self-deprecation.

We are in a creamy sunlit room at the Four Seasons hotel, Beverly Hills, the usual haunt for stellar promotional activity. But right away you know there’s something unusual about Heigl. She looks at you, tries to work out where you’re coming from. I start coughing. She sees to it that hot honey and lemon is ordered to soothe my throat. She doesn’t talk in platitudes. You can see her brain working hard to try to never do that. Her years of getting to the top have been tough. In her past there’s a brother who died after a road accident and a mother who almost died after breast cancer but survived.

Heigl is wearing a tight black dress with a turned-down collar. Her body says Vargas girl, her dress says Doris Day. Her long blonde hair is tucked up into a curiously curled and set faux bob. Is it a metaphor for having it all – long and short at the same time? She laughs at me as if I am mad. “I feel a bit like my grandmother, where you go to the salon once a week and leave your hair for the next week.” She checks it for stiffness, making sure she’s not taking herself too seriously.

She takes nothing for granted. She refers to the past couple of years – which have seen her win an Emmy in 2007 for her portrayal of Izzie Stevens in the hospital drama Grey’s Anatomy, and rock the box office with two hit movies – as “interesting”. “I am very, very grateful.” Then she points out how unbelievable it was a couple of years ago to have heads of studios call her on her mobile to tell her they loved her movie (“Somebody I couldn’t even have got in a room with to audition.”)

It’s a path she’s been treading for 17 years and one that was never dream-easy. She’s now 29. She started modelling at 10 when an aunt, visiting the family in the pleasant suburb of New Canaan, Connecticut sent a picture to an agency. Heigl was taken on by the modelling agency Wilhelmina, and within a few years was off doing acting roles. It wasn’t a smooth trajectory. Projects she chose often failed to make the big screen or were delayed for years. She was most successful as a vulnerable alien in the TV series Roswell (1999), but by that time had also endured the likes of Under Siege 2 (1995), a hopeless Steven Seagal movie, and Bride of Chucky (1998). A thriller called Ground Zero, due for release in autumn 2001, was another casualty of September 11 (it later resurfaced as Critical Assembly). And Illeana Douglas’s directorial debut, Sorority Rule – great script and cast – never saw the light of day.

In short, Heigl has no sense of entitlement, no arrogance. She’s paid her dues. She has a fledgling production company called Abishag Productions. Most Hollywood actors acquire themselves such vanity vehicles, but at least Heigl is sceptical enough to name her business from a line in a Robert Frost poem about the fleeting nature of beauty and fame. I read that men have their turning-point crisis at 40, but women have theirs at 28. It’s called a Saturn return – it’s where women allegedly overhaul their lives and make foundations for the future. Does she feel this happened to her?

“I definitely feel a different person today than I did five years ago. I can’t believe that was me, but at the same time – and this is a contradiction – I feel the same jack I’ve always been.”

What’s different now is that opportunities she once had to work really hard for suddenly unfold effortlessly. There are many choices when before there were none. But she doesn’t want to be smug. “I work hard to constantly change my outlook and not become attached to one idea of where I’m going. I love the idea of having a production company because I love the idea of having some control over decisions and choices that are made. After 17 years as an actor who just shows up, hits their mark and says their lines, it would be great to evolve into having a little more power.” She laughs a cynical little laugh.

Innately she seems to know that power comes from being who she is and not afraid to say what she thinks. While everyone raved about Borat being the most hilarious movie ever made, she announced she found it depressing because it was “mean-spirited” and uncomfortable to look at “Middle America where people still think like that”. In fact, she didn’t laugh at all; she winced and almost cried seeing people be so cruel to one another. She criticised the movie that was her vehicle to stardom, Knocked Up, for being “a little sexist” because the men got all the jokes and the women “looked like humourless shrews”. She can be grateful and critical at the same time, can’t she? It’s just that usually starlets are neither.

Then there was her role in what became known as FaggotGate. Isaiah Washington, then a cast member of Grey’s Anatomy, called his co-star T R Knight a “faggot” before he had come out. Heigl spoke out to the camera at the Golden Globes in 2007, emotionally supporting T R, her friend and groomsman (the male equivalent of a bridesmaid) at her recent wedding to the musician Josh Kelley. People have said how brave it was for her to speak her mind. She laughs at this notion. “Oh, am I supposed to be a bitch now because I say what I think? People have been respectful about it and I don’t know if it’s all that brave or honourable to just be honest. Although at that moment at the Globes I thought I was going to get into trouble for it. I’m sure there will come some day when I’ll say something and I’ll suffer a repercussion…”

But it certainly wasn’t that one. Was it easy when shooting Grey’s resumed after that? Was there an awkward silence? “No, everybody was talking about it. There was tension for a while, but then a lot more joviality and a fresh start.”

Washington, who first denied the slur, then apologised for it, was dropped from the cast.

The show is now in its fourth season, with worldwide top ratings. The figures are 38m for the US alone. Last season saw Izzie, a woman who funded her doctor’s training through lingerie modelling, lose the man she loved to heart disease and then fund a clinic with the money he left her. Suddenly she took on a new side by having an adulterous affair with T R Knight’s character, who is married to her friend. At first she was vociferous in her criticism of her character’s sudden personality change. Now she’s calmer about it, sort of.

“People who are so infallible, perfect and moral tend to be the first to slip and fall. But I would love to see how she [Izzie] deals with the consequences of what she’s done, because what’s interesting is when people make decisions that shake their world, they suddenly have to go, ‘Woo, I didn’t know I was capable of this.’ ”

What has it done to her world view and her belief structure? “I’d like to see Izzie take some culpability,” she says strictly.

Has she ever done anything like that, I ask, knowing she’s far too moral.

“No. I’m not really fond of that. You know, I’m a little black and white, which maybe isn’t fair, it makes me judgmental, but it would be so hurtful to have an affair. You’ve got to be braver than that to the people you love, or maybe loved once. If your relationship changes, the honourable thing to do is end it before starting another.”

Has she ever had to do that?

“Not really. I was never in a relationship and fell in love with somebody else. I always ended the relationship, then met somebody.”

Before Kelley there were certainly other relationships, perhaps most prominently the one with her Roswell co-star Jason Behr, but that seems to have ended with the series in 2002. She tells me how, since then, she has got to know herself better and know more about what she wants and needs. She certainly doesn’t relate from a personal point of view to the emotional withholding she had to do as her character Jane in 27 Dresses – a classic case of “always the bridesmaid [27 times], never the bride”, all the time in love with her boss and unable to say anything about it. “I really didn’t get how she could be in love with him for six years and say nothing at all,” she remarks, incredulous.

In the film she had to dye her hair a darker, duller blonde, and not be diminished by it. It’s a colour called mousy that Hollywood likes to bring out as a metaphor for the overlooked, the “one of us”, not “one of them”. Today Heigl is back to her natural golden with platinum shimmer, but she is still not quite one of them. She is not young Hollywood, rebellious in an attention-seeking way. She is not conspicuously wayward. “I never had any Lindsay Lohan moments. My mother would have killed me.” She never let her insecurities eat her all the way to size zero. In fact, she says she doesn’t have much time for exercise and “I’ve just discovered if I don’t eat bread or pasta, my stomach doesn’t feel so bloated all the time. I try to eat healthy, but I think if you cut out every wonderful thing you love, what’s the point? I’d rather feel sick”.

She’s not so homey that she likes to cook too much. She likes to eat out. “My mother says I should cook more, and she’s probably right,” she says in a way that doesn’t show particular resolve. She doesn’t care about being hounded by the paparazzi. In the past she said she doesn’t like actors who moan about the paps. Recently, when moving into her marital house, they were all lined up outside. She and her husband asked if they’d mind helping the process by moving some boxes. They obliged.

Did working on 27 Dresses prepare her for her own highly orchestrated event? “Planning my wedding was definitely stressful. The seating chart! The calligraphy!” she grimaces. It was a winter-wonderland wedding in Park City, Utah, what they now call a destination wedding, so skiing could take place if desired. She looked gorgeous in her Oscar de la Renta dress. It was fitted and frothy. “It had a corset for sure. Anything I can keep in place I will. Old Hollywood is my favourite kind of look. I want to be able to look back on the pictures 10 years from now and still love them.”

More importantly, she wants to look back on her husband 10 years from now and still love him. She’s serious about love. They met when she was cast in a video for his 2005 single, Only You. She says, in her early twenties, she was more of a people-pleaser, more the kind of girl who would play it cool and wait in a traditional girl way. When she met Kelley, she didn’t bother with that kind of cool. She thought they connected right away. She didn’t wait for him to call. She called him. She was sure from the start.

“He was everything I was looking for, or needed or wanted in my life. I’d been through enough relationships to understand myself better. I knew what it was I needed from someone so it’s not a waste of everybody’s time.”

And what exactly was that?

“He’s just so easy to be around. He lightens up the room and he lightens my load.”

She talks about how easily she gets stressed and how he helped with that.

“From early on in the relationship I felt I could really trust and feel safe with him, and feel loved by and laugh and laugh and laugh with him. That’s really important to me.”

I had read that at first he was quite hesitant, quite cool. “He was, yeah, I think about that sometimes, and how mysterious he played it.

I would have been that type of person but I decided I was sick of trying to figure out what everybody else wanted – I should just decide what I wanted. He was not being emotional right off the bat, and I was used to having the reverse of that. Perhaps that’s why I was blown away. I’m usually much cooler.”

So were you easily able to be this other version of yourself? “Yes, it was kind of ironic. It was like, ‘Ooh, I don’t know if I like this,’ but I did,” she says.

It’s clear that a relationship that flowed with such comparative ease was new for her. They were together a year, then got engaged and were engaged for another year and a half.

“I thought that was the right time because I wanted us to grow as a couple so we’d be ready for marriage. Also I thought it would take a long time with our schedules to prepare the wedding. By the time it got to it I was, ‘Let’s just do this.’ ”

There’s a curious, old-fashioned quality to her. The idea of not living together before marriage appealed to her. I tell her that is scary. Her parents converted to Mormonism when she was seven – more of that later – and although she is no longer a fully practising Mormon, “there is something of that still in me. I wanted to keep a bit sacred”.

They moved house to the same area in the Hollywood Hills. He would come and stay with her while they were dating, but he had also been touring for a lot of that time, so until now they had no experience of even a joint washing cycle. She says she thought about just staying in that house and redecorating “to make it feel like ours, but I also felt it would be easier and more exciting to start afresh”.

Kelley proposed to her by renting the same bungalow by the beach where they’d first met to make the video. Cute. Is she planning on instant babies? “I think we’re both on such a career trajectory right now, it would be foolish to waste the opportunities. I think he’d prefer to wait a little more time, but I kind of wouldn’t, so I think we’ll meet somewhere in the middle.” She smiles with such certainty and accuracy you know it’s something that has been discussed, argued, agreed. Yet fitting into someone else’s life must have been a complete upheaval when she’d lived on her own for so long.

She lived with her mother, Nancy, until she was 22. Her mother is now her manager. They speak every day at length. Her mother used to be a housewife until she was divorced in 1996 from Katherine’s father, Paul, a financial adviser. Heigl says the divorce was not unexpected: they’d all seen it coming for years. What was a problem was, that same year, her mother got breast cancer and had a mastectomy. Heigl was relentless in her support, saw her mother through chemotherapy, and was with her every day. She said it was a real wrench to move out.

“We had such a great time together, it seemed a no-brainer to live together. But I did reach a point where I felt I needed to be on my own.”

She dedicated her Emmy win to her mother and has said in the past how her mother didn’t come to LA to make friends. “Her job was to protect me and be fierce in defending me. This is a fear-dominated industry, it’s about rise and fall, and my mother refuses to be intimidated by that.”

Heigl also refuses to be intimidated, or believe in her good or bad luck. There has always been a misfit quality about her. At school she was often working, acting or modelling, and didn’t have a huge number of friends. She didn’t like hockey or cheerleading, was surprised and delighted when she found something she was good at (acting), so she carried on working.

She does try to connect with you and doesn’t just talk about herself, but when she does, she gives you emotional minutiae. She’s the youngest of four siblings. She has an older brother, Holt, and an older sister, Meg. She was only seven, out shopping with her mother, when her 15-year-old brother, Jason, was involved in a fatal road accident. He was in the back of a pick-up truck. A girl who hadn’t had her licence for long was driving. It was raining. The car spun out of control. The accident happened 40ft away from their front door. Upon arrival at the hospital, Jason was braindead. His family donated his organs, and Heigl is still a passionate supporter of organ donation. At that time, Katherine’s mother was Lutheran and her father Catholic. The tragedy led to their conversion to Mormonism because a couple of Mormon families were a great comfort to them. “I give the Mormon church a lot of credit for helping them,” says Heigl.

Did having her brother die in an accident right outside her house affect her in that “wanting to seize every moment” kind of way? She looks at me knowing perhaps it should, but says the opposite.

“For a long time it made me afraid of being too happy because things happen so unexpectedly. Your world can change and every aspect of your life is different – and not necessarily for the better. So it made me afraid when good things happen. It made me brace against it that little bit. I’m trying hard not to do that any more. It doesn’t make something terrible easier, and I don’t want to waste my happiness.”

So were you able to stop thinking in that negative way as soon as you felt successful?

“No, it got worse. I had to think something’s bound to happen that will make it not so wonderful any more. Then I had to tell myself to stop being so dramatic.”

She says this as if she’s still working through it, and that only increases her likability factor. It’s a subtle, buried thing, but being worried about it all being taken away deletes smugness from her life. So what about the rumours that she’s now too big a star to return to Grey’s Anatomy?

“When I come back to Grey’s, I am actually really excited. After nine months a year for four years, of course you’re going to think, ‘I want to be someone else for a while.’ I landed really good things and I did them in my breaks. It broke up the monotony of the character and I’m very, very grateful for opportunities. But Grey’s is such a good cast. Everybody is amazing,” she says with a wide-eyedness that would look false on anybody else.

“I’m really, really lucky,” she says. And you want her to be.

27 Dresses is out now. Grey’s Anatomy is on Five at 10pm on Thursdays

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