The Dark Knight of Ledger's soul

Heath Ledger — who died as the result of an accidental prescription drug cocktail in January — was something of a self-taught "method" actor, immersing himself in his roles. It was a habit he picked up from his Oscar-nominated break-through role as a gay cowboy in Brokeback Mountain, and an approach he continued through parts as diverse as a drug-addicted artist in Candy, the villainous Joker in The Dark Knight and even a circus performer in Doctor Parnassus.

Early amateur stage work and TV appearances in his native Australia had brought Ledger to Hollywood. But packaged as a lightweight "hunk" to appeal to teens in comedies 10 Things I Hate About You and A Knight's Tale, Ledger soon rebelled. "I was being offered a deal with the devil," he said of the his early successes. "I felt professionally cheapened."

To escape the teen-idol tag, he took smaller parts, such as Billy Bob Thornton's son in Monster's Ball, before returning to Australia to work with his friend, the director Gregor Jordan, on a film he hoped would rehabilitate his ambitions, playing the title character in Ned Kelly. During filming, Ledger began an affair with his co-star, the actress Naomi Watts, who in turn brought his attention to the role of Ennis Del Mar in Brokeback Mountain. For the first time, Ledger threw himself body and soul into a role, carrying aspects of the character into his real life, and turning inward around friends. It was a habit that was to contribute much to his mental detriment.

For the role of the Joker, the implications for his mental health were particularly serious. Part of Ledger's preparation was the compilation of a "inspiration book" he called "The Joker's Diary". He filled it with thoughts and images he felt might help flesh out the Joker's story, including a list of terrible things the Joker might find funny, such as Aids. "I locked myself away, experimented with voices," said Ledger. "I ended up landing more in the realm of the psychopath..."

His Joker was to be a monster with no redeeming features. "He's just out of control," said Ledger. "He's a sociopath, psychotic, mass-murdering clown." As the film's director, Christopher Nolan, said: "He's inhabiting the character in much the way I'd hoped from a psychological perspective. He's created something quite terrifying."

By this time, Ledger wasn't much enjoying life at all. He'd met actress Michelle Williams on Brokeback Mountain and they'd had a daughter. But the pair were now separated. The official reason was pressure of work, but rumours were rife of Ledger's womanising and drug-taking. The biggest problem for Ledger was his separation from his two-year-old daughter. He'd revelled in the role of new father, and now he'd lost that opportunity to worry about someone other than himself.

His despair deepened with the knowledge that playing the Joker would put him back in the intrusive media spotlight. Intensely private, Ledger had had a running battle with the Australian paparazzi – who he'd tried to escape by relocating to New York – and he also lacked confidence in his own performances, fearing he'd fooled audiences and critics.

Struck by anxiety-ridden insomnia, the 28-year-old turned to prescription medicines to help him sleep. His journeys around the world to shoot movies — at the start of 2008 he was hopping between London and New York — meant he drew on the services of several doctors who didn't know what the others were prescribing.

Ledger's death, it seems, was a tragic accident waiting to happen – but it may just be that the Joker was the prime suspect who finally drove him over the edge.

Brian J Robb is the author of 'Heath Ledger: Hollywood's Dark Star' (Plexus, £12.99)