The real exorcist helping Hollywood banish its demons

When Sir Anthony Hopkins agreed to add to the list of terrifying characters on his CV by playing an Italian exorcist in the horror film The Rite, he decided that a bit of actorly research might be in order. So he whisked Father Gary Thomas away from his comfortable home in Silicon Valley, California, to spend a week on the movie set in Budapest.

Father Gary is a bonhomous 57-year-old who ministers over the Sacred Heart parish in the town of Saratoga, a suburb of San Jose. But outside his day job of saying Mass and hearing confession, he boasts an intriguing second career: as California's leading campaigner against demonic possession.

One of about 50 Vatican-certified exorcists working in the United States, Father Gary has seen it all: people speaking in tongues, rolling their eyes, hissing, spitting and taking on the characteristics of a serpent. Often, his clients will throw their bodies into wild contortions - though if you mention The Exorcist he'll reply that he's never seen a rotating head. Sometimes, they even try to assault him. "I get attacked by demons all the time," he says, casually tucking into a lunchtime turkey sandwich at his home this week. "They don't scare me. They shouldn't scare me, because although I believe in Satan and the personification of evil, I also know how to defeat him."

Unlike most of his peers, whose trade has been shrouded in mystery and superstition since the Dark Ages, Father Gary is happy to talk publicly about his battles with darkness. This has turned him into the nearest thing the Roman Catholic Church has to a media exorcist. It also saw his life story chronicled in The Making of a Modern Exorcist, the book by the US journalist Matt Baglio on which The Rite is based.

Sir Anthony and the film-makers invited Father Gary to Budapest to ensure that religious scenes were depicted accurately. He trained the cast in everything from the correct way to deliver exorcism prayers ("very forcefully") to the proper technique for wielding holy water, a crucifix and a stole. "They were very emphatic about making sure the movie was accurate, and it is, it really is," he says. "I never get offended if someone doesn't believe when I tell them what I do. But I can assure you that during my career I have seen everything that is depicted in those scenes, except for one part where a character regurgitates iron nails."

During his time on set, Father Gary became close to both Sir Anthony and his wife, Lady Stella, who were concerned about the possibility of facing a demonic attack during filming ("I told them it could happen, but don't be afraid. Stay close to God"). He believes their conversations helped to strengthen the Welsh actor's faith.

"Tony was delightful. I can't say enough good things about him. He's so down to earth. He wanted to know what my own take on exorcism is, how I got into it, whether I believe in Satan, and how I know he exists. He's a recovering alcoholic, and said to me, 'I used to be an atheist and then certain things happened in my life.' So now he considers himself a Christian. I found him to be very sincere."

Father Gary didn't fall into the business of battling demons until 2005, when his bishop asked him to spend a few months training in it at the Vatican's Athenaeum Pontificium Regina Apostolorum in Rome.

Exorcism may feel outmoded in the modern church, but belief in the existence of Satan (and therefore his ability to occupy a human soul) remains central to Christian theology. Today, there are practising exorcists in roughly half the Catholic dioceses in the world. Though the church is reluctant to release exact figures, there are about a dozen in the UK.

During training, Father Gary gained practical experience as an assistant to Father Carmine De Filippis, who is played by Hopkins in the film. In the few months they worked together in Rome, he witnessed roughly 80 exorcisms. Since returning to the US in 2005, trade has been less brisk: he has seen roughly a hundred potential clients, but performed exorcisms on only six of them.

"The hard part of my job is knowing when someone actually requires an exorcism," he says. "A lot of people call up and say, 'Father, I need an exorcism.' I reply, 'It's not a tetanus shot! I don't do them on demand.' I have to first discern whether they are actually possessed. That's what takes the real time. Most aren't, of course. For most, it's mental health stuff. We try to help with that, too."

Father Gary works closely with a doctor, a psychologist and a clinical psychiatrist. Over several interviews, he'll look for behaviour his medical colleagues can't explain, including "manifestations", or physical signs that a demon is present. Sometimes his subjects display an aversion to going into church, or being touched with holy water. Other times, they'll start contorting their bodies or rolling their eyes during prayers. The exorcism rite, if required, is relatively straightforward, though it usually has to be performed several times to achieve success. Father Gary reads from a text asking a demon to reveal its name, and then commanding it to leave. There are relatively few pitfalls, he says: "But you must never start conversing with the demon. That's the classic mistake."

Father Gary's most recent exorcism occurred just a fortnight ago. His subject was an immigrant from El Salvador who. like roughly 80 per cent of the people he sees. had been abused as a child.

"I hadn't even started the prayers and he suddenly belched and said, 'Don't you want to know my name?' Then his voice changed, he began rolling his eyes," he says. "It was scary for the man's friends, who were watching. But I just told them the same thing I always say, and the thing you'll learn from watching The Rite - 'Don't be afraid, this is about faith. Stay close to God.' And we did."

- INDEPENDENT