WHY, you wonder, was a book called Self Hypnosis For A Better Sex Life on American actress Rosario Dawson’s reading list? It was nothing personal, she says, just one of many on the subject of the mind that she probed preparing to play a hypnotherapist in Trance, the new thriller from Slumdog Millionaire and Trainspotting director Danny Boyle.

“There are all these really interesting books out there about how the mind works,” she says, the morning after the London premiere of the film.

“I like reading that stuff anyway and was interested to get into that world. I went to a California institute and they said that within six months of training, I could be doing free hypnotherapy sessions with people.”

Dawson is not an actress who feels every role must be thoroughly researched, but she wanted to feel as comfortable as possible as hypnotherapist Elizabeth Lamb, who becomes involved with thieves in the aftermath of an art gallery heist.

“In this case it was really helpful to have as much background information as she had, because that’s her toolbox,” explains Dawson, whose previous movies include Sin City, Men In Black 2 and He Got Game.

“Every time danger signals flash, she steps forward because she has some knowledge about how the mind works and how she can play the situation to make another move to keep her safe.”

Together with other cast members James McAvoy and Vincent Cassel, she had a session with a New York hynotherapist. “But we’re all control freaks, so no one actually wanted to go under,” she recalls.

“I did have a session on my own with a woman in LA. It was really basic, she didn’t get me to walk around as a chicken or anything. It was just lying back and throwing a towel over me. Your temperature drops because you go into that sleepawake kind of space.

“You’re listening to her voice and I presented something I wanted to work on. I felt my legs spasming and that jerky feeling you get sometimes when you’re falling asleep. Then afterwards she just read me. She could see where I was at. I was like wow.

“I thought that was phenomenal.

Our subconscious wants to be heard so it speaks to us all the time – when you have fantasies, are dreaming or have nightmares. If you know how to read them, it can be very helpful to you. If you don’t, you’re missing a large part of your psyche or personality.

We are mostly our subconscious, way more than we are our conscious mind.”

Before the research, Dawson thought this sort of mind-play was a bit of a parlour trick. “Now I’m really glad I had such a positive experience and understand the science behind it,” she says.

“The people I met help people with very serious issues that they’ve had for 20 years, like phobias and bad habits and trying to quit smoking. If you get a really good hypnotherapist, you can really change your life.”

Therapy hasn’t appealed to Dawson before, but then, she says, hypnotherapy doesn’t feel like therapy.

“It’s not going to be me sitting there divulging all my secrets. I don’t really want that. Forget my past, what I’d like is to do something for my future.”

Not only her mind, but Dawson’s body was a key part of her performance, notably in a full frontal nude scene. “It was not expendable, it was not something that would be cut, or something to negotiate,” she says.

When she read the script, she knew there was no point auditioning if she wasn’t comfortable with the scene. “So I’ve long been fine with that idea,” she says. “You go there and you know you’re doing something quite risky. Hugely so.

“Now I’m watching the film and excited and thinking this is all right.

I’ll be happy to see this is my later years and go, ‘I’m so glad I had the guts to do that’.”

  • Trance (15) is now showing in cinemas.