Altered images   Marc Evans' psychological thriller Trauma, starring Colin Firth, has found a perfect home in the Gothic vaults of St Pancras Chambers. The director and star tell Mark Salisbury how the location affected their vision

As spooky locations go, St Pancras Chambers is among the spookiest. With its vaulted ceilings, peeling wallpaper, long, dim corridors and dank basement, the Victorian Gothic building—once the Midland Grand hotel, now Grade I-listed—reeks of must and menace. A perfect place, in other words, to film a psychological horror chiller such as Trauma.

Which is why, on a cold morning last June, Colin Firth is skulking the building's corridors looking more like a tramp than the heartthrob that Pride and Prejudice made him. Dressed in scruffy jeans and jacket, he looks dishevelled and downbeat, playing a character who believes he is haunted by visions of his dead wife (Naomie Harris) who died in the car crash that he survived. He may also be a murderer, responsible for the death of a pop star whose body was found washed up in an east London canal.

Equal parts Don't Look Now, Jacob's Ladder and Patrick McGrath's Spider, Trauma revolves around Firth's grief-stricken coma survivor who, attempting to get on with his life, moves into a flat in a converted hospital and is soon befriended by a neighbour, Charlotte (Mena Suvari from Americans Pie and Beauty), who is into crystals and all things spiritual, and who tries to help Ben find some peace. But as his tenuous grip on reality starts to loosen, we are left wondering not only about Ben's mental wellbeing, but whether the ethereal Charlotte may be another figment of his disturbed imagination.

"The beginning of the film is about grief, then changes to being a film about madness," says Trauma director Marc Evans, who helped start a mini-revival in British horror cinema with his last film, the inventive thriller My Little Eye. "It's about loneliness, about someone whose grief kind of leads them to a kind of madness and a warped perception of the world."

As the first film to emerge from The Ministry of Fear, the production company headed by former Edinburgh Film Festival director Lizzie Frankie, and devoted exclusively to horror—whose slate includes projects penned by Muriel Gray and novelist Kim Newman—many eyes are on Trauma to see whether it can deliver. Yet Evans doesn't see Trauma as being a horror movie per se. "It's definitely in the zone of grown-up psychological thriller," he says. "It's a film that takes itself seriously. It's not ironic. It doesn't play jokes with an audience. It starts with a concept: what if you lost your wife and you woke up on the day that the world was grieving for a Princess Di or Jill Dando, some kind of celebrity grief phenomenon?"

"It is much more about mind games and paranoia and what might be frightening and what might be menacing," says Firth, during a break in filming. "There will be a bit of boo as well. It's unashamedly trying to mess with your mind a bit."

So how does Firth go about playing a man who is not in control of his senses? "If you're playing a character who can't distinguish reality from fantasy, you have to use your judgment," he explains. "If something seems real to you, you have to play it as if it's real. So in some ways it's perfectly simple. He thinks his wife's dead and then he sees her; thinks maybe she's alive, but he's not sure. You have to think yourself into that situation; it can be a fairly freaky thing. You certainly can't play a thing called madness, because nobody thinks they're mad."

To help realise Trauma's altered sense of perception, and to drive home the sense of unease, Evans was keen to use odd, unsettling locations and show a view of London that hadn't been seen of film before. "We were trying to create a certain idea of Canary Wharf and the City as this kind of malign, faceless modern East End, and then below that this kind of Gothic, almost Victorian place where Ben might live. The idea is he moved into a flat in a converted hospital, and maybe in a strange way he's never left the hospital during the whole film." Such a place initially proved difficult to find. "We found a lot of interesting interiors but I kept feeling we didn't have the scale of, say, the Dakota Building in Rosemary's Baby."

Evans eventually found what he was after in one of London's most well known landmarks—St Pancras Chambers. As the Midland Grand hotel, which opened in 1876, it had been famed for its then innovative features, among them its hydraulic, ascending chambers and revolving doors. Closed as a hotel in 1935, it remained in use as offices until the 60s when it was listed and saved from demolition, although it lost its fire certificate in the 80s and has been empty ever since. Although the exterior was renovated in the mid-90s, the interior has been seriously neglected - and this has proved popular with filmmakers, with The Madness Of King George, Richard III and even the Spice Girls' Wannabe video among those having filmed inside. For Evans it provided the perfect architectural feature for his film. "We wanted a really long corridor because all films about people going mad have got long corridors in them," he laughs. "There's one upstairs 250 feet long."

According to Firth, the building is more than simply a visual metaphor for his character's descent into madness: its unsettling ambience has even seeped into his performance. "It's doing all the work today as far as I'm concerned," he says. "It looks paranoiac, if you light it right. So in many ways these are my days off. It's very rare that as an actor, you get any of the stimuli that your character would get, but they've managed to make the atmosphere so creepy at times."

The building was always been one of his favourites, Firth admits. "I'd been dying to get inside. It kind of surpasses expectations because it is just as kind of gloomy and ghostly as I'd hoped it would be, but is also more magnificent than I imagined. You can see squares where nasty paint has been taken off and underneath there's a piece of extraordinary Edwardian wallpaper." Of plans to renovate the building as a luxury hotel, he says: "In a way it's almost a pity to do anything with it. It feels like what it must be like if you could go down on the Titanic."

For Firth, Trauma was an opportunity not only to work with Evans again—the pair previously collaborated on the 1994 Ruth Rendell TV adaptation Master Of The Moor—but to shed the labels of period and romantic comedy, for which he has become synonymous for a while. "It's the kind of film I love to go and see, and I haven't spent a lot of time doing the kind of films I love to go and see," he reflects. "There have been a lot of romantic comedies made, and a lot have come my way, but I never go to them. This is something that interests me. It reminded me a little bit of some of the paranoia films that I liked in the 70s, some of the Polanski films, and things like Don't Look Now."

Like his star Evans acknowledges the shadow that Don't Look Now, the Nicolas Roeg classic, casts over Trauma. Both films deal with the supernatural and the effects of grief. Both films trade in Gothic menace, a fractured narrative, the use of fragments of colour and cracked imagery to shock and dislocate. Both films rest on the reliability of the narrator. But one is a certified classic and the other is only released this month.

"It's almost as unhelpful as it is helpful to have a film that good as an influence, but it definitely inspires, to be in that territory," Evans says. "I've kind of made myself a rule of never invoking other directors because you're never going to be as good as them necessarily. Having said that, it's very hard to have a conversation about genre films without talking about other genre films. That's the difference between them and art movies. No self-respecting art moviemaker would say they were trying to make a Tarkovsky film. Inevitably there are connections between genre films and there's a kind of British tradition you can aspire to, [which] Roeg represents par excellence."

It was Evans' unfamiliarity with the genre that made My Little Eye so refreshing, as he brought an unsullied perspective to a well-worn formula, and made himself something of a star in the horror field as a result. So even if Evans doesn't see Trauma as a horror film, that is how it is going to be perceived. Is he comfortable being pigeonholed as a horror director?

"What I'm comfortable with is being allowed to work," he says with a wide grin. "After House [Of America] and Resurrection Man I literally couldn't get arrested. I'd committed some crimes against filmmaking, as perceived by financiers; I had made films that were dark and didn't make any money. The great thing about genre filmmaking is that for someone like me who's not interested in social realism or social comedy, it's probably one of the places I could exist and be employed, because it requires imagination and intelligence and to play with ideas in a cinematic way. Financiers don't look at you askance because you've gone a bit weird on them. They want you to be weird. If that could be my only resting place, I'd be happy with that. If that meant I could make a film a year, I'd be very happy with that."