Meryl Streep and Alec Baldwin are sitting on identical high-backed chairs in a reception room at the Plaza hotel, just off Central Park, in New York. Streep is sipping a gin martini, “Very dry, with a twist”, which seems perfect; Baldwin’s “water and an iced tea” less so. An Irish whiskey might have been more appropriate. She’s in a black dress, he’s in a dark striped suit. It’s early evening, between the red-carpet premiere and the reception for their new film, It’s Complicated. Baldwin, 51, is big and loud and expansive, with a fabulously rich, deep voice that can quickly fall to a seductive growl, and an infectious Irish love of the grand story, true or not. Streep is effortlessly regal, a touch prickly, acidly witty, giving as good as she gets, often dissolving into hoots of laughter.

It seems astonishing that Streep, 60, is at the pinnacle of a career that has had many pinnacles. She has won two Academy Awards — best supporting actress for Kramer vs Kramer in 1980, best actress for Sophie’s Choice in 1983 — and has received 15 Oscar nominations, more than any actor in history. Who doesn’t think she’s the greatest living American actress?

With nothing left to prove, she is having a ball, throwing caution to the wind in films such as Mamma Mia!, the musical that became one of the most successful films of all time in the UK; The Devil Wears Prada, playing a deliciously icy facsimile of the American Vogue editor, Anna Wintour; and Julie & Julia, for which she looks certain to get another best actress Oscar nomination as Julia Child, the large, and larger-than-life, doyenne of American cuisine, with Stanley Tucci as her bald and much shorter husband. In real life, as they say, Streep has been married for 31 years to Don Gummer, a sculptor, with whom she has four grown children.

The very least one can say about Baldwin is that he is the best actor on American television today. His Emmy wins in the past two years — as outstanding lead actor in a comedy series, playing Jack Donaghy, an egotistical right-wing tele-vision executive and Tina Fey’s boss on 30 Rock — testify to that. Yet Baldwin is a man of powerful appetites and heady, often contradictory, ambitions, recently judging his whole film career a “complete failure” and vowing to give up on Hollywood because he despises every movie he has ever been in, and himself in them. You must have impossibly high standards to see Glengarry Glen Ross, or The Cooler, for which he won an Oscar nomination, as failures. Still, tonight, for my benefit — actually, for that of Streep, whom he obviously reveres — he’s making a damn good show of having enjoyed doing this film. In his real life, Baldwin is known for his brutally public divorce from the actress Kim Basinger. He has fought a long battle for time with their daughter, which has made him a vocal advocate of fathers’ rights.

It’s fascinating to see two such different actors, such different people, working together for the first time. It’s Complicated, a romantic comedy, has a slew of refreshing twists. Streep plays Jane, the owner of a fancy bakery and restaurant. Baldwin plays Jake, her ex-husband, a charming, roguish lawyer. They’ve been divorced for 10 years, ever since Jake ran off with a much younger woman, to whom he is now married. Jane and Jake have three grown-up children. After years of barely speaking to each other, Jane and Jake find themselves unexpectedly spending the evening together out of town, at their son’s graduation. After they have knocked back far too many drinks, the evening turns into a passionate, sweaty night. Before they know it, they are having a full-blown affair.

It’s Complicated is written and directed by Nancy Meyers, who has carved out a unique niche in Hollywood with romantic comedies about, and for, real — ie, older — people, including What Women Want, starring Mel Gibson as a man who can hear what women are thinking, and Something’s Gotta Give, in which Jack Nicholson finally ends up with an age-appropriate partner, Diane Keaton. One of the main pleasures of It’s Complicated is watching older — well, somewhat older — people having a proper physical, sexual relationship in a big studio movie. Yes, there’s flesh: more than a hint of Streep’s, and a barrelful of Baldwin’s. The two of them spark off each other so well, it seems best to give you their patter straight, as it happened.

Christopher Goodwin: The conventional wisdom in Hollywood is that films for older audiences, particularly for older women, don’t make money.

Meryl Streep: Oh, do we have to talk about conventional wisdom? Why are films about young people for all people, when films about older people are only for older people? Why is that? That’s a more interesting question. You know what? A laugh is a laugh.

Alec Baldwin: I feel funny because yes, I’m older than I was, but people say “older” like I’m George Burns or Walter Matthau.

Streep: And he’s way younger than I am. But things have changed. Bette Davis was 40 — 20 years younger than I am — when she made All About Eve. And she was saying she was washed up. She took the famous ad in Backstage — “Actress. Multiple Academy Awards. Looking for work.”

One of the moments in It’s Complicated that gets tremendous laughs is when, after you’ve started having an affair with Jake , you say to your girlfriends: “It turns out I’m a bit of a slut.”

Streep: It’s because I’m saying, “We’re not done, baby! Don’t underestimate me.” Why are we consigned to the rubbish heap at a certain age?

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