Heath Ledger's final cut: An exclusive, on-set diary

I'm breathless and Heath Ledger is downright furious. He rips off his frilly clown hat and hurls it to the floor. It's a minute past midnight and the cameramen are looking at their watches and mumbling stuff about "the union". The director Terry Gilliam is beside himself too, as he scrambles around the set of The Imaginarium of Doctor Parnassus to persuade his mutinous crew to agree to one more take; but it's too late – the permit to film ran out at midnight, and pieces of equipment are already being hastily stashed into their silver flight-boxes, ready for the next job.

In a final attempt to salvage the situation, Heath joins Terry in petitioning them: "C'mon guys... Please! Just one more take... Just one more. I mean, c'mon, what difference is another 10 minutes going make?" But it's all in vain as they continue packing.

As it will transpire, the scene that's just been shot – a vile mob giving chase to Heath Ledger through the winding backstreets of London's East End – will be the last he'll ever shoot. I was a member of that mob; and in roughly 72 hours, Ledger's dead body will be found by his personal masseur in his loft apartment in New York City.

Nearly six months after his death, as the PR machine for The Dark Knight swings into gear, the actor will unavoidably be in the spotlight again; there's also a suggestion that he'll be nominated, posthumously, for the Best Supporting Actor Oscar, for his role as the Joker in that film.

The first time I saw a publicity still from the Batman film, it was a disconcerting experience: a live man, looking like a dead man already. Heath Ledger as the Joker, with lax, yellow hair, caved-in face, and smudged, blood-red lipstick, bright and colourful and terrible, like a Japanese water-demon, or something from a Corman horror. This image, loaded with ghostly resonances, speaks solely of death; and this is what I find strange, because in those last three days I worked with Heath, I encountered a man who was so full of life.

This was my first job as an extra, and on the first day, before I'd even had the chance to sit down and quaff a quick coffee, we were given our call and escorted down to the set: a tatty and forgotten pub in the heart of Clerkenwell – The Ring O Bells. Terry Gilliam was dashing about, a hand on his battered, suede cowboy hat, to stop it flying of his head; in his wake, a small retinue of production minions struggling to keep up with him. The willowy and strangely beautiful Lily Cole was making her way across the set, and as if from nowhere, a tall, thin figure appeared and pranced and jigged his way towards us – it was Heath and he was dressed up like some daft and dishevelled Pierrot doll.

"Jesus! Heath, you look crazier than a clown's cock!" I offered. He creased up with laughter.

"And.... CUT," shouted a distant voice; then "Good... Good... We'll go again in five..."

"That's hilarious," said Heath. "Where'd you get it from?"

"A film called Kenny," I told him. "A mockumentary about this guy who's got a Portaloo business in Melbourne".

"Oh, Jeez... I know the one you're talking about, it's got what-his-name in it? Shane Jacobson – that's it! Shit, I really must get to see it..."

And with that, Gilliam beckoned him over to the monitors. It was soon apparent that Heath was utterly immersed in this role and in this whole project. After each scene had been shot, he'd be running off to watch it played back, regardless of whether he had starred in it or not. He was so active on set that if he wasn't wearing such an outlandish costume, it would have been impossible to distinguish him from the any of the production team's top brass.

All the talk on the set of was of his performance as the Joker. The buzz was that once it was released, Heath would to be seen in a whole new light – as a "proper" actor, a "brilliant" actor, possibly. He would be massive – absolutely massive; and after what I'd seen of his work ethic on that first day, absolutely wasted too. '

The following day, I happened to arrive at the unit base at the same time that Heath and his PA pulled up in some outrageous super-car a certain German manufacturer had loaned him while he was staying in London. The roar of the engine drowned out my quick "Hello", so I nodded casually and walked straight past, headed for the catering truck.

I popped back after lunch to have another look at the car. As I inspected it, I noticed Heath sat on the steps of his trailer, a black hoodie pulled tight over his head, skinny black jeans and a pair of sneakers, and sucking on a fag as usual. After a minute or so, he wandered over, his PA lurking behind him carrying his Starbucks bucket and Camel fags. "So what d'you think of the car, mate?" he asked.

"I'm not too sure, cars aren't really my thing, but I know what Freud would say..." I replied.

"It's ridiculous isn't it? Talk about a cock-extension... Ha! It's fun, but not really my style," said Ledger. But he seemed a bit uneasy and broke off the chat, saying something to his PA. They wandered back to his trailer together.

Back on set, Terry and Heath were soon having another of their private conversations. It was hard to tell who was directing who. I shimmied closer, only to overhear some scurrilous gossip about Tom Cruise. Heath eventually broke off and came over to ask if any of us had seen the new film about Joy Division – Anton Corbijn's Control: "Their music's amazing!"

On the final day of filming, Saturday 19 January, there were guns and explosions and violence on set. There were arguments, and a bad vibe descended on the pub. Heath himself no longer looked like a clown. He was dirty, wired and manic: he hadn't stopped for three days – kicking about the set whether or not he was due to shoot a scene. He'd be there when I arrived and after I'd gone. And I was doing a 10-hour shift. When he wasn't on set he was back in his hotel room reading or watching some of the Oscar-nominated movies that, as a member of the Academy, he'd be asked to vote on.

He'd been throwing himself around a lot, doing his own stunts, take after take – attempting to lob himself on to the "Imaginarium", a horse-drawn, travelling sideshow, decorated with a series of Gilliam's own hallucinogenic graphic confections – sometimes succeeding, sometimes failing.

It was common knowledge on set that he had a spinal injury and that he was on some hefty medication for it. Late in the day, with shooting behind schedule, Heath's back was playing up. He lay prostrate on the cobbles between the pub and the Imaginarium doing his Alexander Technique exercises, motionless, his eyes shut tight. As I walked past I nearly tripped over one of his elongated clown shoes.

"You all right, Heath?" I asked.

"Yeah. I will be in few minutes..."

"I thought you'd snuffed it there," I said, trying to raise a laugh. Heath just closed his eyes. Once he had recovered, the filming resumed; Ledger pursued by an angry, drunken mob, all baying for his blood. Incendiary devices were popping everywhere, fired from a blunderbuss by Verne Troyer, the 2'8" actor who played Mini Me in the Austin Powers films. And this is the last scene Heath Ledger ever shot; it reached midnight and the union curfew kicked in.

By the time Heath and Terry calmed down, the set had thinned out dramatically. Heath walked around, thanking and hugging people, then came over to us few extras who were still left and thanked us and began walking off. I walked after him to ask if he was going to stay and have a few drinks.

"Sorry, but I'm on the wagon... have been for about 17 months now," he said, mock-triumphantly

"Oh... nice one!" I replied, somewhat tongue-tied.

"Cheers, mate" he said before turning and sloping off despondently up the narrow lane back towards unit base and his warm trailer. "Bye Heath..."

The following Tuesday, at about 8pm, I received a text-message from my sister, who I'd been keeping in the loop regarding my adventures on Doctor Parnassus. In that dull and toneless medium, and in the truncated vernacular of text-speak, it read: "Wot sort of effect do u have on people? U no that actor u were workin with... they found him dead!"

It took a while to register, then I turned on the radio and, within seconds of finding a news station, her message was legitimised: "Heath Ledger... found dead... being treated as a possible suicide... slumped on the floor of his loft-apartment in New York..." I called a couple of other extras to find out if they knew what was going on. All they knew was what I knew: Heath was dead – the circumstances open to speculation. They all expressed a sense of shock and loss. Some wept.

As I sit here, looking at his picture, I still really don't know what to say about Heath Ledger. All I can add to what's already been said is my imperfect but valid little story: the story of a man whom I met, but whom I never really knew; the story of a man who I worked with for just three days but left one of those indefinable imprints that make you feel you've known someone a lot longer.

My image of Heath is of a man envisioning a life rather than a death; of an actor deeply committed to his art – perhaps to such a degree that it contributed to his undoing. But looking back at my time on set, I also see strange portents of his demise: there was even a moment when one of the extras, a devout Christian, began reading aloud from The Revelation of St John. And after our conversation about Joy Division, whenever I think of Heath, I'm reminded of the band's lead singer, Ian Curtis – another young man with immense energy stubbed out in his prime. Heath would have liked such a comparison, I think.

This wasn't how the movie was supposed to end; I was shocked, I still am; but then, what do I know? I was just an extra.

The writer's name has been changed for the sake of anonymity. 'The Dark Knight' (12A) goes on release in the UK on 25 July