Shia LaBeouf. (WENN)

Actor Shia LaBeouf bulked up his slender frame by 50 lbs. to convincingly play Tom hardy’s onscreen brother in new gangster film Lawless.

The Transformers star admits he felt puny compared to muscly Hardy when he first met with his co-stars, so he hit the gym hard and trained intensely for two months to look the part.

He weighed in at 180 pounds when the cameras began to roll and LaBeouf admits his physical transformation was aided by the fact that The Dark Knight Rises villain agreed to drop a few pounds too, so they would "level out" with their other movie brother, Jason Clarke.

LaBeouf tells ETonline.com, "There's a fabric of the family that needs to exist. So had I showed up in my fighting weight – which is 130 (pounds) – next to Hardy I wouldn't have looked like I was related. So I shipped out at (bulked up to) 180 (pounds) and he dropped his weight. And Jason upped his weight and we all tried to level out."

The trio play the bootlegging Bondurant brothers in the prohibition-era film, which also features Gary Oldman, Guy Pearce and Jessica Chastain.