Shia LaBeouf had to bulk up for his role in 'Lawless'.

The 26-year-old actor was instructed to pile on the pounds for the Prohibition-era gangster movie and he admits it took him some time to get used to his new muscles.

Shia said: ''I had to gain 40 pounds. That was probably the most physically arduous thing I've done, you know? I've never had that kind of weight on my chest before. It's something to get used to.''

Shia stars alongside an A-list cast comprised of Tom Hardy, Gary Oldman, Jessica Chastain, Mia Wasikowska and Guy Pearce, but admitted the most intimidating thing about the role was learning to drive the vintage cars on set.

He explained: ''It's not like you've got a Clutch and a throttle - it's totally different; you've got pedals that you've never seen before.

''You look down at the floorboard - there are 12 pedals there. You think you're flying a hover board. These old-school trucks were very difficult to drive.''

Harvey Weinstein, whose production company is spearheading the film, revealed he is proud of Shia, who he mentored to Hollywood success.

Speaking at a 'Lawless' screening in New York last night (14.08.12), Harvey said: ''Matt Damon and Ben Affleck and I gave birth to Shia. You've seen that John Ford movie, '3 Godfathers'? That's us. We did a TV series ... that was called 'Project Greenlight'. And one of the first movies that we made was on that show, and Shia was the actor. I am so, so proud of him, and so are his other two godfathers.''