Outside Johnny Depp's Winnebago the discarded props of a working film production await collection at the end of a day's shoot. There's a box jammed with wigs next to a rail lined with velvet frock coats, a stack of buckled shoes and an array of feathered hats.

Nearby, though, precariously positioned against a wall, is unmistakable evidence that The Libertine is quite unlike most ordinary period dramas.

"Have you seen the giant phallus?" Depp inquires cheerily, pointing to the bright-orange object in question, which must be 8ft tall. "It's an impressive, er, prop, don't you think?"

This, after all, is the story of John Wilmot, the second Earl of Rochester: poet, wit, war hero and exponent of debauchery on such a scale it would make your hair curl more than the jumble of hair pieces nearby. There was nothing ordinary about Rochester.

Depp is clearly having fun. As are the crew. They inform me that I've chosen the wrong day to visit the set. "You should have been here yesterday," says one. "Makes the long hours worth it, mate."

Rumours of the explicit content of Laurence Dunmore's film have spread far from the town of Ramsey, in the north of the Isle of Man, where Depp, John Malkovich (who plays Charles II) and co have been based in the studio for the past few weeks. Indeed, when the production filmed in England and Wales, at historic locations including Hampton Court, there were times when the film-makers were hoping that they weren't asked too many questions about the script.

Some might have been a little anxious when they discovered that Rochester's life was something akin to a 17th-century rock star - only with more sex and drink. "If you want a modern comparison, he was as famous as Mick Jagger," says writer Stephen Jeffreys. The producer, Russell Smith, adds: "He was like a rock star. Actually, he was more like a punk because he put the finger up at everybody."

A legendary hellraiser who wrote excellent, often exceedingly filthy poetry, a wit who amused and outraged the court of Charles II, Rochester bedded just about every woman he came into contact with and drank far too much before he died, from syphilis, at 33.

"The location we were most afraid of was Wells Cathedral because they hadn't asked anything at all and we were using the Bishop's room as our House of Lords. And we were like, 'Oh my God, if they read this script we are out of here so fastI' We started filming and rumours were spreading that the movie had bestiality in it or something, so we were tiptoeing around that one. But we went and showed them that we are regular people and they were fine."

Jeffreys' script, adapted from his own stage play, portrays Rochester as part hedonistic hoodlum, part tortured soul who was at turns capable of expertly lampooning London society, penning near-pornography chronicling his latest bawdy exploits, and writing some of the most beautiful prose of the time to the women he genuinely loved, his wife Elizabeth Malet (Rosamund Pike) and his mistress, the actress Elizabeth Barry (Samantha Morton).

"The Libertine is about someone who has a fantastic talent for everything and he wastes it," says Jeffreys. "It was deliberate, destroying barriers, knocking things over. I think it was partly in response to his revulsion at the court of Charles II, although I have the feeling that whatever the society he would have been against it in some way."

Depp was hooked from the moment he read the opening, "You won't like me" monologue that Rochester delivers to the audience at the start of the film.

"That was it for me," he says. "It was one of those rare occurrences where you read a script and you think, 'This is great.' Three sentences into the opening monologue and I was in. I knew it was one of those things, the kind of material that you see just once." He also agrees with Jeffreys that beneath the punk exterior, there was a sensitive soul struggling to be heard. "For all his adventures and all of his sexual exploits and deviant encounters, he was quite sensitive and loving," says Depp. "When you read the stuff, he was a very caring man. And a great, great writer."

Right now, Depp is finished for another day. He offers a glass of red wine as he chats about the film and wardrobe arrive to take away his costume.

Outside, the nondescript walls of the Isle of Man film studios are providing the backdrop to that prop. It's a surreal sight. "I think they should keep it there," grins Depp. "Like a tourist attraction. 'The Earl of Rochester was here.'" n martyn palmer The Libertine opens on November 18

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