It may be cold during the chilly Sundance Film Festival, but elegant Tilda Swinton remains in a perennially chirpy mood. No stranger to this mother of all Indie festivals in the thick of Utah's Park City, Swinton was here to promote Thumbsucker, one of two films in which she appears with Keanu Reeves, the other being Constantine. "Yes, we were talking about having it written into our contracts that we will always work together," she says, laughingly.

Though not participating in the print press junket for Constantine, Swinton, who at the time of our interview hadn't seen the film, says she has reason to be optimistic about this latest comic book adaptation which is already generating early buzz, having no qualms about setting foot into the world of mainstream Hollywood, but has a blasé attitude when it comes to worrying whether her films will be widely seen. After all, Swinton is not afraid of doing a Teknolust on the one hand, or a Constantine on the other. "I'm very lazy about people seeing my work. I'm an arrogant believer in the power of films to find itself an audience, and a bad studio film will actually get stumps in a video shop, and is going to reach less people I haven't done many studio films to be honest with you, but a Derek Jarman film that runs for 20 or 30 years is going to reach more people so the reaching people is something that I feel very apathetic about."

But Swinton said she "enjoyed the adventures of both my latest Hollywood films," referring to Constantine and the upcoming Lion, the Witch and the Wardrobe she completed for Disney. "In both instances, what was exactly the same was that I went into these adventures because of the film makers. Francis Lawrence [Constantine] blew me away when I met him and he was an extraordinary individual. It was fantastic working with him, and both Francis Lawrence and Andrew Adamson, are incredibly experienced. Both of them are first-time film makers actually, because when you think about it The Lion, the Witch and the Wardrobe is Andrew's first live action, so again, there I was, business as usual, working with a first-time film maker, going on this adventure and with that sort of beginner's mind, and that's what I love. I love working with film makers who have that absolute sort of mad, kamikaze aspect to them," she enthuses.

In both films, Swinton plays extreme fantasy characters, the Arch Angel Gabriel in Constantine and the White Witch in Wardrobe. The actress insists that she sees no difference in playing those kinds of characters and more realistic women she has portrayed in the likes of The Deep End or Thumbsucker. "It's all about imagination because they don't really exist. Audrey in Thumbsucker doesn't exist and the woman in The Deep End don't exist, because they're all constructs and so what you're doing when you're playing a character is just making a sort of shadow play for the camera about the person because you can't actually follow a person all the time. All you're going to show is a series of details which is going to mean that the audience can project onto the screen their idea of who that woman is. So when you're acting, you're not actually doing anything real at all, so it's the same whether you're playing the Angel Gabriel or the White Witch."

Swinton was nit keen to give to much away about her Angel Gabriel. "There really is so little that I can say about the Angel Gabriel because the Angel Gabriel's been kept in this kind of surprise pocket in the film," hence her decision not to do a lot of press for the film. But working not once, but twice, with Keanu on two such different projects, was a revelation, she says. "I don't know that anything exactly surprised me about Keanu when I first met him. I think he just endorsed what I thought he would be which is that he's like an Angel actually, so open and so up for company, which is what I always thought he would be, and that's exactly what he is."

Swinton is equally excited about her participation in Disney's The Lion, the Witch and the Wardrobe. The actress spent 5 months in New Zealand shooting the film "and had a fantastic time." Swinton says she is genuinely convinced that fans of the classic books won't be disappointed in this big-budget screen adaptation. "I'm really convinced that it will be exactly what it should be, which is a classical, very cinematic adaptation of that book. It's got a really 3D Technicolor, Wizard of Oz feel about it and one of the things that I think is very radical about what Andrew's done, is he is not interested in special effects anymore so everything's real, so you have real creatures. In other words, you don't have, as in Lord of the Rings, 500 extras that are doubled up to make 7,000. but actually have all of those people in all of those suits, being all of those mythical creatures." Swinton says "it was really good fun, and I really, really love New Zealand."

All of this is a far cry from her home in Scotland, where she would like to spend some more time working, the last time having worked on Young Adam. "Of course I would like to work more there and constantly am working with a variety of Scottish film makers to make those happen but it's a slow process." Dividing her time between making British films and those of various budgets across the Atlantic, Swinton is both star and executive producer of Thumbsucker, a critical success at Sundance which was just picked up for US distribution by Sony Pictures Classics later this year.

A film about an adolescent thumbsucker, petrified of independence, Swinton plays his mother who has her own thumbsucking issues. Swinton was ferociously drawn to the film, she says, because "I saw something really special in director Mike Mills, who, for those who don't know, is a very well-regarded graphic artist, who has made documentary films and has made some of the most beautiful music videos. So I was aware of his work anyway and was always interested in his aesthetics but when I met him and he talked about what he wanted to do in the film, the kind of atmosphere he wanted to create, I totally fell in love with it. It was very difficult to get the film made and we found it very hard to get people to give money for it."

But the film did get made and attracted quite the cast. Swinton sees this as being more than your typical coming-of-age story. "I think a lot of coming of age stories focus on the impossibility of communicating with parents, the idea that the parents know exactly what they're doing, and that the only person who has any growing to do is the child. However, in my experience of life, that is just not the case, and I think that's the most beautiful thing that this film does. I mean, who is the thumb-sucker here? Who is the one who needs to separate? Who needs to grow, who needs to bawl like a baby on someone's shoulder? I think that it's a coming of age story in as much as a coming of age for the parents as it is for a coming of age for the boy. It's just that the crucible of the plot if you like, is around him separating and going away to college."

Next up for Swinton, is "a film with a Hungarian master Béla Tarr, called The Man from London which is based on a Georges Simenon novel. It's a sort of European co-production - French, German, I think there's some British money in it. It's about a man who witnesses a murder and it's deeply existential. Béla Tarr is quite extraordinary, a sort of Tarkovsky of his day and I hope he'll forgive me for describing him as that but he's one of the great masters working today. We're shooting in March and I'm going to be developing a variety of things this year."

The 43-year old Ms Swinton is on a roll, it seems, one that takes her literally to hell and back. It's quite the ride.