On a sultry Friday afternoon, movie-star charisma bowls into the room, felling all before it, beaming, charming and trailing its owner a few paces behind. Christian Slater’s LA people have insisted he be chaperoned throughout our interview; and though he is more than capable of running any show, you can understand their caution. For a young man of

34, he has plenty of past to be cagey about, which makes his apparent openness today doubly winning.

He arrives hungry and exhausted but determinedly upbeat, in a grubby T-shirt and jeans, baseball cap pulled down, slouching low in his armchair. He is here to rehearse the roguish RP McMurphy in One Flew Over the Cuckoo’s Nest, Dale Wasserman’s adaptation of Ken Kesey’s black comedy, grandfather to all the wayward, borderline-nutty kids Slater has played. After a staging at the Edinburgh Festival, it will transfer to the Gielgud Theatre in London’s West End, where its star has been in rehearsal all day, bonding with his new director, Terry Johnson (the project’s instigator, Guy Masterson, having withdrawn citing personal reasons).

Clutching a sandwich and a pack of cigarettes, Slater chuckles and snorts at jokes and forgives your interruptions, but there is an air of unpredictability about him, a back-burning fierceness that means you can feel his handshake long after it has ended. When he smacks a fist into his palm to swat a fly, it stings the air. He is short and sweet, but make no mistake, he likes to be in charge: testosterone flows like Jack Daniel’s (I should add that he is drinking cola), igniting the room in precisely the same way that McMurphy rouses Nurse Ratched’s rubber-soled regime of sadism and sedation. Indeed, Slater seems so irreversibly awake that I find myself asking him how he ever gets to sleep. He looks dead serious: “I have to lie there and force myself to keep my eyes closed.”

He had never heard of the Edinburgh Festival before being offered the part, but he knows the importance of this story, whose lead character became the anti-hero of a 1970s generation beginning to question who was more insane: the inmates or their institutions. Slater has made troubled young men his forte — Jason Dean in Heathers, the cultish Clarence Worley in True Romance — but McMurphy is, finally, truly heroic. “I always seem to find these quirky, odd, off-centre characters,” he says in his loud, unfaltering voice, “and this one is the pinnacle of them all: a big hulking lumberjack full of love, full of life. He comes into the neutral routine of the hospital with his nudie deck of cards and shakes the place up.”

Slater’s whole life has been acting: there was never a thought that he might do anything else. He saw Richard III at five; Shakespeare in the Park; his father, the actor Thomas Knight Slater, took him backstage when he was playing Dr Watson in Sherlock Holmes on Broadway. He was discovered on the late-night Joe Franklin talk show, where his mother, Mary Jo Slater (then an agent, now a powerful casting director), was appearing as a guest and he was brought out as a novelty. The next day, he was called in to audition for The Music Man, and then whisked away on a nine-month tour, on which his busy mother could not accompany him. When he started acting, he switched to the Professional Children’s School and began a childhood of touring, tutors and surrogate mothers. The comforting cocoon of the baby performer kept him wrapped in candyfloss, but also forced him into adult concerns before he was ready, a conflict he believes may have contributed to his subsequent troubles.

Playing the novice monk opposite Sean Connery in The Name of the Rose in 1986 made his name in Europe, but he recalls it now as thrilling but painful exposure for a shy teenager. When asked to perform a sex scene, the only advice the seasoned star could give him was to keep breathing. “Oh, God,” he groans in memory. “It was humiliation everywhere. I was an adolescent trying to deal with changes in my body, and they shaved this friggin’ bald patch on the top of my head and then told me to do a love scene.

I mean, I was 15 — they’d be arrested if they tried to get a kid that age to do it today.”

They gave him a copy of Umberto Eco’s novel to read, but he told them they had to be joking. He never got around to it. Does it sit on his shelf, taunting him? “Nah, not any more. I got rid of it.” Slater laughs as he describes the overwhelming abundance of desirables — drink, girls, and premiere-pampering — ushered in by that film, but it was also the beginning of a desperate time.

Having spent so much of his childhood being cute with Dick Van Dyke in The Music Man, or adorable as Oliver in Kansas City, by the age of 18, he was courting oblivion with drugs and alcohol. He was arrested twice for drink-driving, once for carrying a loaded gun onto an aeroplane and once for assault, after which he spent

59 days in prison six years ago. Was he high and happy, or wretched during his public meltdown? “I was having a great time,” he smiles, “but also going through a personal battle, dealing with issues about growing up and being in the business and becoming a man, struggling with the changes.

I hit some walls and speed bumps ... I had moments of sheer embarrassment. I look back and see my life has been a wild ride, and to be here now, strong enough to do this play, is amazing.”

The shame of his public disgrace stopped him in his tracks.