Helen Mirren is trying to understand the concept of all-consuming egotism. She is sitting in her trailer in Vilnius, the sedate capital of Lithuania, outside what was once a Soviet sports centre, in between shooting scenes as Elizabeth I for Channel 4’s forthcoming epic. There she is grappling with the problem of how to convey Good Queen Bess’s apparently awesome self-regard. You would think a lifetime in show-business circles would have provided perfect preparation for this one. But no. Elizabeth’s self-centredness was of an altogether different order.

“I still can’t quite get to grips with it,” she says. Divested of her jewel-encrusted finery, but with her face made up and her arms folded over a plain dressing gown, Dame Helen awaits the arrival of a rushed lunch of beef stroganoff before normal reigning resumes in the afternoon. “Elizabeth had absolute certainty that she was in the right place,” she continues. “No flattery was too much for her — the level she could stand was nauseating. The ego was huge.”

As, indeed, were those clothes and that hair — or “the gear”, as Mirren calls it. For two months, the ugly expanse of the sports centre was turned into a temporary sound stage to play host to a beautifully dressed re-creation of the glittering Tudor world at Whitehall Palace. It was populated by numerous, somewhat perplexed-looking, and presumably cheap, Lithuanian courtiers. (“Elizabeth didn’t mean anything to them,” she says of the local extras. “We kept having to say, ‘Very important queen.’”) Written by Nigel Williams, the two-part drama covers the second half of her reign and focuses on her romantic attachments to the power-hungry men around her, concentrating particularly on Robert Dudley (played by Jeremy Irons) and the Earl of Essex (Hugh Dancy). Any parallels with DS Jane Tennison, Mirren’s hugely popular Prime Suspect character, who will be returning for a seventh outing next year, are, however, smartly dismissed. “You cannot make those analogies, you really can’t,” she says brusquely. “Elizabeth is closer to Margaret Thatcher, if you want to find a modern analogy. She believed she was chosen by God; not like Jane Tennison, who’s there because she’s ambitious.”

Weeks later, and with filming over, we meet again in London. Elegant in black mufti, Mirren turned 60 a week ago. She looks no more than 45. I ask her whether she had Elizabeth’s God-given gargantuan self-esteem nailed.

“I don’t know whether I got to grips with it, but I got used to it. It’s what people always say about playing kings and queens: you can’t play it, it’s the people around you who make you into what you are, in their attitude to you — in acting terms, I mean.” She has certainly done her fair share of rulers, from Cleopatra at the Old Vic at just 18 to her Oscar-nominated Mrs King in 1994’s The Madness of King George. Before the year is out, she will be adding yet another to her list, about which more later. She continues: “But the woman inside that queenly thing, the ego, was a different matter. You just go from day to day, scene to scene, play it in the best way you possibly can and hope for the best. I haven’t seen any of the material at all — I don’t watch dailies (the previous day’s unedited footage) unless there’s a specific reason to.”

The roll call of actresses who have played Elizabeth I stretches from a silent Sarah Bernhardt to the Oscar- nominated Cate Blanchett and the Oscar-winning Judi Dench. The fascination seems endless: Blanchett will be reprising her portrayal in the forthcoming sequel to Elizabeth, The Golden Age. And, in an odd stroke of timing, Mirren’s mature queen is being shadowed by a rival BBC production featuring the younger ER, played by Anne-Marie Duff from Shameless, which is due to air early next year.

The interpretation that left the most lasting impression on Mirren before she took up the baton, however, was Miranda Richardson’s spoilt, squeaky schoolgirl in Blackadder II. “Obviously, it’s comedic and cartoony, but you can see the elements of truth in it,” she laughs. “She was obviously, as a person, very emotional — she cried, she screamed, she fainted — so we do try to express that.” And did she end up liking her? “Oh, yes. But I felt I would have been frightened of her if I’d happened to be in the same room as her. She gets angry a lot — she obviously had that thing a lot of people in great positions of power feel, of becoming addicted to getting angry and making everyone quiver. But then she was terribly vulnerable: Essex only had to go off and sulk for three days and she’d be giving him anything he wanted. She could be emotionally manipulated like that, very feminine. You don’t get the sense of a big butch dyke striding across history.”

For Tom Hooper, the director of Elizabeth I, Mirren captures such extremes of emotion particularly well. “With lesser actors, you risk going over the top incredibly quickly, but she has the ability to take extreme positions and root them, make them feel incredibly real. One of her great gifts is that she carries a tremendously strong sense of reality with her as an actress.”

That sense of reality is present in Mirren’s face. The famously earthy sensuality comes with a reassuring, almost domestic familiarity and, paradoxically, a slight hauteur.

Perhaps this mixture comes from her background — her grandfather was a White Russian (her real name is Ilyena Lydia Mironoff), her mother a working-class English girl from Pimlico. Whatever its origins, the range it produces is evident in the way she can glide from upmarket gangster’s moll (in The Long Good Friday and The Cook, the Thief, His Wife & Her Lover) to middle-England comedy (Calendar Girls) to Shakespeare’s heroines. And, as with all the best actors, there is an inherent sense of danger about her.

Mirren is married to the director Taylor Hackford and based in London — but with a house in Hollywood. She has had a reputation as a rebel, a hippie chick and the thinking-man’s sexpot, uninhibited about disrobing for her art. All of which makes her next role, on the face of it, an unlikely one. With the first Elizabeth in the can, she was, when we met, preparing to play the second, in the much-anticipated film about the tumultuous weeks immediately after the death of Diana, Princess of Wales, in 1997. The Channel 4 film is being made by the team that produced the Blair-Brown television drama The Deal, and directed by Stephen Frears; it will depict the relationship between the Queen and Tony Blair at a time when the monarchy looked as if it was seriously faltering.

There are piles of videos to watch and books to read, and Mirren will be working with a dialogue coach to capture the Queen’s distinctive world-weary voice. But there is already a surprising physical resemblance there. “I look terribly like the Queen, actually,” she says. “If I had to wear a dark wig for a role, I used to look like Princess Margaret. I haven’t put on all the gear yet, but I’m sure that the minute I put on the Queen’s hair, I ’m going to look very much like her.”

Given the monarch’s almost impenetrable public face, Mirren is aware that attempting to portray her “from the inside” is going to take a huge leap of imagination, although she is scouring the tapes for moments that show the real woman behind the image. In a way, she says, Elizabeth I allows more freedom for an actress.

“With Elizabeth II, she’s alive, she’s with us, she’s incredibly important to us — it’s a really different thing. I think all I can do, really, is be me as her. I cannot be her. I’ve never lived that life, I don’t know what it’s like to be woken by bagpipes every morning, to wear the most beautiful underwear all my life, to never have a run in my stocking — although I suspect she does, actually, because she’s not into clothes. And that’s another fascinating thing about her: her utter lack of vanity, her utter lack of interest in what she wears, which is alien to me, because I love clothes.”

While not sentimentalising the Queen, the film is, says Mirren, sympathetic to her position at that time “in an objective way”. The hysterical reaction to Diana’s death certainly polarised opinion: you wept or recoiled at the weeping. “I’m not a monarchist, but I’m kind of with the Queen on this one,” she continues. “I’m not sure I get it, either. I wasn’t in England at the time, so I was blissfully separated from it. I was quite grateful that I wasn’t in England, actually.” Her own publicised anti-monarchism seems, anyway, to have mellowed with time; the years, she says, have brought a respect for the values of the Queen’s generation, and she even shares the perception that she has a growing, new-found affection for the monarch herself. “I do. I feel that in myself.”

We have been talking within walking distance of Buckingham Palace, and the streets are thick with police. It’s exactly four weeks since the bombings and London is experiencing a seizure of high security. Before we wrap up, I ask Mirren for her take on the current situation. She ponders for a while; she’s not sure what she thinks. She went cold when she heard about the bombs while in Lithuania. “I guess my take on it is that you can make anyone believe anything — and especially men.

“Sometimes I deeply hate men,” she says, half-laughing. “They’re so gullible sometimes, and so stupid a lot of the time, young men especially. I see it slightly in sexual- politics terms as much as political politics. I’m utterly confused about faith in general, because I don’t have any, and I can’t bring myself to have any, as much as I try. I can’t get my head round it, so I can’t get my head round people needing to have it. Yet I know that all cultures in the world have faith, different faiths.” What you can try to understand, she continues, choosing her words carefully, is poverty and exclusion, and discrimination towards women within certain communities. “I believe,” she says, “that everything is down to the education of women.”

Elizabeth I might well have approved of that.