Slowly and steadily, Laura Linney has risen through the acting ranks, taking leads in low-budget indies and solid supporting roles in expensive Hollywood productions. With two Oscar nominations on her CV, she now finds herself firmly on the 'a' list. But as the actress tells Hugh Linehan, movies were an afterthought she always assumed her destiny was in theatre.

I GET sent a lot of stuff that's just so bad, that it's just not actable," says Laura Linney. "Scripts are written to be greenlit, they're not written for the actor, particularly Hollywood scripts. This movie was challenging in ways that I don't normally do. Also, there was a great cast, a good group of people."

We're sitting in a midtown Manhattan hotel to discuss the 41-year-old actress's latest film, The Exorcism of Emily Rose. Based (very loosely) on the events surrounding the death of a young woman in Bavaria in the 1970s, the film, transposed to the US, stars Linney as a religiously agnostic attorney assigned to defend a priest (Tom Wilkinson) accused of negligent homicide in the wake of the death of a young woman. The priest believes she was possessed by demons; the medical establishment believes she suffered from a form of epilepsy and should have been handed over to its care. Events leading up to the death are retold over the course of the trial.

It's the sort of genre mash-up - "The Exorcist meets The Verdict!" - which one imagines might play better at pitch meetings than on the big screen.

"All of us were concerned with how you combine these two genres together," Linney admits. "The suspense of a good courtroom drama with the fear of the supernatural. A lot of the time, when you have two powerful genres they cancel each other out. How does the suspense of one heighten the fear of the other? How does that fear heighten the intensity of the courtroom scenes?"

In these times of religious fundamentalism in the US, with science and reason on the retreat in the classroom, one might also be a trifle concerned at any suggestion that the supernatural should receive any credence in a court of law, or that certain religious beliefs should be shown as exempt. Could the same principles be applied to female circumcision, or honour killing?

"I was very concerned when I first got the script that this was not going to do that," says Linney. "It was imperative to me that it should be shown with an even hand."

Well, yes, but on the one hand we have Linney battling with her own doubts and beginning to accept the notion that there really is something spooky going on. We also have decent old Tom Wilkinson as the priest, with Campbell Scott as the nasty prosecutor and a clutch of arrogant and incompetent medical types batting for the Enlightenment. Sounds like a loaded argument.

"I don't think it does do that," rejoins Linney. "It's based on a true story, and for every argument that one side has, there's an equally strong argument from the other side."

But there is an implication towards the end of the film that the young woman may not only have been possessed; she may be a saint. "Or she may be insane," replies Linney. "The priest is charged with negligent homicide. The question is not, what is she possessed by? Clearly she's possessed by something, whether it's psychosis or a religious demon. I know that people can be possessed by envy or greed or lust, which are personal demons. Is it possible for someone to be possessed by a religious demon? I'm completely sceptical about that."

In truth, The Exorcism of Emily Rose doesn't hang together well enough to merit more than a footnote in the history of the Culture Wars. But you can see why the part appealed to Linney: it's the kind of meaty yet ambiguous role she has specialised in over the course of a varied career which has spanned theatre, television and film.

The career has been on an upward trajectory in recent years, especially since her Oscar-nominated performance opposite Liam Neeson in Kinsey, and her Lady Macbeth-ish partnership with Sean Penn in Mystic River. Earlier, there was her turn as Jim Carrey's "wife" in The Truman Show. And she had previously been nominated for an Oscar in 2001 for her terrific work in the winning independent movie You Can Count on Me.

"It's all been good," she says. "The past two or three years have been particularly good, starting with The Crucible on Broadway, then Kinsey and Mystic River."

Along the way, Linney seems to have cornered the market for a certain type of role: women who may appear the epitome of conservative domesticity on the outside, but who have darker currents running beneath. Does she search for that ambiguity when choosing her work?

"I think the one thing I know is that no one is just one thing, and people will surprise you," she says. "You can be the nicest person in the world and you treat your mother like shit. There are all sorts of contradictions which make the person what they are. I don't do it intentionally, to trick people. Most of the time it's in the script somewhere. If it's not in the script, even if it's very quietly, then it's not going to work."

Laura Linney was born in 1964 into a New York theatrical family, so it was not surprising that she followed a career in acting. "I really thought I would just be a stage actor," she says. "I thought I'd live in New York and do theatre all my life. Film and television has been the great surprise of my life. It used to be stage, stage, stage, because it's what I grew up with and was comfortable with. But the more I do film the more I love it. And I've made more than 30 films now, much to my surprise."

She agrees that You Can Count on Me was a turning point for her, after playing a string of supporting roles in films such as Absolute Power and Primal Fear.

"There are a lot of jobs that can change your career because they change other people's perceptions of you. But then there's the jobs where you actually really grow and take a huge step forward. On You Can Count On Me the conditions were so hard. We had to shoot so much, and so fast. But I had done enough film at that point to have some confidence and relaxation with the actual work, that I could just access.

"When you do film, nobody teaches you anything. And no one has the time to teach you. It's not that they don't want to, they just don't have any time. So you have to pick it up as you go along. Congo, as silly as it was, I love because it gave me an opportunity. I was on that movie for six months. It was the first big-budget movie I was on from beginning to end. There was no acting involved, but what I was able to do was spend three weeks with each department. Whether they were aware of it or not, I would hang around with a department and find out what it was they were doing. Who are these people?

"If you grow up in the theatre, you know who everybody is and what their job is. I can tear a theatre apart and put it back together. When you walk into a film crew and you're not from that world, there are so many people that you wonder what's going on. There are cables and there's people called gaffers. And what's a focus puller? You don't understand any of it when you're thrown into this different world. What's really exciting is when you understand what a camera crew is doing and how you work with them."

Working with Clint Eastwood on Mystic River and Absolute Power was also valuable, she says. "I learned a lot from him about how to stay warm right throughout the day. You have to be very careful when you arrive on the set in the morning and say hello to everybody. You can waste a lot of energy in the hair and makeup trailer and you're exhausted before it's 7.30 in the morning. You've been up at four, and by 11 o'clock you're ready to go to bed and you've still got 10 more hours to go. You're disoriented on a movie set.

"On this movie, we'd start at five in the morning. People are not designed to act at that time. I think I gave my opening statement in the courtroom at four o'clock in the morning."

Eastwood's strength as a director, she says, is his simplicity. "He has complete confidence and trust in the people who he works with, and it just happens. And he doesn't feel he has to cover everything from nine thousand different angles. Peter Weir's the same. There's nothing nagging at him about 'I've got to do this to please this person or that person'. There's no wasted footage."

Linney has cropped up in big chocolate-boxy movies like Love, Actually, but most of her recent work has been on more independent-minded projects, although she's not impressed with the direction some of the indie companies are taking.

"They've had some successes and now they're straying from what they know to be true about putting together a good movie," she says. "They'll have gorgeous scripts, but now all of a sudden they're concerned about box office in a way that they weren't when they made their really good movies. And that's why those movies were good in the first place, because they weren't worrying about a result. But now they have requirements about casting and other stuff."

So does she see herself moving more into the mainstream? "I'm realistic about it," she says. "And I know if I want to have a family at some point I'll have to make some money in my life. The indies are the best thing in the world, but they're hard. It's one thing to give up your life and be paid for it. It's another to give it up, be fulfilled artistically, and just scrape by. So there are trade-offs.

"Occasionally you have to give yourself a break and do what you have to do. But the three independent movies I did over the last year had been trying to get off the ground for the last four years. I committed to doing those movies, I attached myself to them. And then they came up. So I couldn't say no."

She is, she says, feeling the stress of being on location all the time.

"Being a gypsy is really hard. I love it, but I was on location for 48 out of 52 weeks last year. I made four movies in four different countries. I was in my own bed for 10 nights last year. There's a perception that the more successful you get the easier it gets. But it's more complicated for me."

She stops herself and laughs. "I know how lucky I am. There are people who would chew off their arm to do what I do, but it does get overwhelming. And there is nothing worse than a complaining actress. Nothing worse than a successful, complaining person. I find myself complaining, and I'm saying 'shut up!' to myself."

The Exorcism of Emily Rose is released next Friday