You get the feeling a lot has to do with being typecast decades beneath her age. "I played teen roles until high definition came out," Blair half-jokes (I later realise this is true), "and I could never understand it. I would go in for adult roles and be older than many of the people auditioning, but they'd cast the girl without a line on her face." She shrugs. "I suppose I might have seemed too troubled or cynical to play a mother. The closest I'd get was an adult who was emotionally unstable or drug-addled, which is about 17 years old emotionally anyway."
She talks about the fact that her CV comprises two extremes: "very light" teen romances such as The Sweetest Thing ("where I sort of do Technicolor, almost theatrical") and "very dark" roles (the Hellboy trilogy). Her latest film, the aptly named Dark Horse, directed by Todd Solondz, falls into the latter category. In it Blair plays the highly medicated Miranda, who meets a troubled 30-something (Jordan Gelber) at a wedding, but the dysfunctional pair are denied any sort of predictable trajectory. The appeal was reuniting with Solondz, who directed Blair in 2001's Storytelling.
"He's such a remarkable, sensitive director, but it was understood that I would not be in another film of his because he only really works with people once." He decided differently when Blair had dinner with him - "we were talking about my career and he gave me hope. And you know, he has helped change my outlook. I feel totally committed to having a wonderful career again."
She explains how the film lifted her out of a "black hole" that was as personal as professional after she got a divorce from her husband of two years, Ahmet Zappa - son of Frank - in 2006. "It felt like failure, as if I hadn't thought things through. And I honestly didn't try very hard to make it work," she confesses. Getting Dark Horse was "a real shot in the arm. I did this TV show, Kath & Kim [the US remake of the Australian hit, with Molly Shannon], and I felt badly about it. I was overeating to play this character and I think I lost sight of a lot of things."
Falling pregnant with her son Arthur during filming (with her now partner, fashion designer Jason Bleick) also "kickstarted something. It was like I had a baby and I suddenly started to feel I could play anything. Just as my CV has "light" and "dark" sides, I suppose there are two sides to my own character," she adds. "I go from being hugely hopeful and entertaining to ... really not. I'm not manic depressive, but I can really go to the darker side."
Blair's other new project, the hotly anticipated TV series Anger Management, based on the Jack Nicholson film, is perhaps her most high-profile to date, not least because it marks her co-star Charlie Sheen's comeback after a very public unravelling. She laughs at the irony of playing Sheen's therapist.
"He had a huge moment last year, but I do think he's a great actor," she says. "And he's very present in this - he's been incredible on it. He's kind and affable and loyal to the people in his life. But you know, I don't think we'll ever know about the stresses someone is under that would make them speak out so loudly."
In light of this, she views her own level of fame rather gratefully.
"Perhaps I have managed some sort of longevity because I haven't won the lead roles," she says. "I don't have the pressure of being a world-famous bombshell that has detonated." We joke about how things could have been different if she had not suffered early rejections, such as not getting the lead in Buffy the Vampire Slayer [it went to her Cruel Intentions co-star Sarah Michelle Gellar] or Joey Potter in Dawson's Creek. "There was me, Katie Holmes and Marla Sokoloff up for it," Blair explains, "and Katie got it, which was absolutely right - she was perfection. But I could've been living in a castle right now, with Tom Cruise as my husband."
Would she like that? "I would, because I have such a crush on him! Not as big a crush as I have on Bill Nighy. But I wish someone would jump on a couch for me ..."
Blair's career choice was perhaps surprising to her single Jewish mother, a judge who was, she says, not very maternal: "She scheduled C-sections to have each of her four daughters at 8.45am on a Friday morning and was literally back at work on Monday morning. But since becoming a mum myself I've begun to appreciate that more."
Blair is the youngest of the four and grew up in suburban Detroit. After getting a degree in English, psychology and photography at the University of Michigan she relocated to New York to become a "photographer or actress, whatever hit first". Was her mother supportive? "She helped me pack my bag, although there wasn't really any financial support [Blair lived with the Salvation Army]. But I think she did believe I could do it."
Blair was spotted by an agent while taking acting classes at the Column Theatre. A small part in the 1997 film In and Out followed. "I had to audition six times for it, and I ended up with two lines that were cut," Blair remembers, laughing, "but I got to spend six weeks in the Catskills with Kevin Kline and Joan Cusack, who is my favourite. I felt like an actress."
Now, she says, "you have to do it all to make a living, unless you're Cate Blanchett. You have to write five books and come out with a line of socks.
"I got approached to do a reality show not so long ago." And? "I decided I'm too much reality for anyone. I don't have the gaggle of goofy friends, and you have to do a lot of branding."
Besides, she is entering a new era now. "I am hopeful again. I want a good life and a great career more than I ever have. I don't know that I actually believed I deserved one until now."
When and where: TV2 Wednesdays 8.30pm
-Observer
