People will admire Jude Law's "chutzpah" for taking on a role in which he has "mutton chops" and a "big belly", the film's producer said.
Law plays the lead in Dom Hemingway, a film about a man back on the streets of London after spending 12 years in prison - and the star even took time on set to advise his co-star on love.
Jumayn Hunter, 23, from Tottenham in north London, plays bad guy and Law's enemy Lestor, and said the star was "one of the coolest people" he has ever met.
The young actor said he "nearly fainted" when he got the role, but before long he found himself on set taking tips on love from Law.
Hunter said the Cold Mountain star's top advice was: "Be yourself to the best of your ability."
Producer Jeremy Thomas said cinema goers will love Law but will not see him in the same way as in his previous films.
"They'll love him. You've never seen anything like it before. He put on an enormous amount of weight. He's got mutton chops and he's got a big belly, and he's very full of expletives and swearing and bad language and bad actions," he said.
He added: "You admire the chutzpah to play a part like that and to take the bull by the horns. He loved doing it. He read the script and said immediately 'I'm doing it'."
Speaking at an event at the BFI on London's South Bank looking ahead to the Toronto and Venice film festivals, Hunter described Law as "focused", but keen on having a laugh too.
"He's very focused, very dedicated and doesn't like many distractions, but in between shots and in our spare time he's the first person to crack a joke or whatnot," he said.
He added: "Jude turned out to be one of the coolest people I've ever met in my life. We're very similar in very many ways.
"We just got on like a house on fire. We bounced off each other, threw ideas at each other, played around with the characters a lot until we found something that we both liked and just went with it."
Dom Hemingway also stars Richard E. Grant and will be screened at the Toronto film festival before hitting UK cinemas in November.