The Zombieland filmmaker admitted he was so nervous about directing "the world's greatest actor" that he couldn't sleep before Sean's first day on set.
"I didn't get a lot of sleep the night before Sean's first day," he said.
"I was definitely nervous about directing one of the world's greatest actors, who is also a great director in his own right and has made more movies than I have."
These nerves proved to be unfounded and the 38-year-old director found Sean, who plays maniacal Mafia leader Mickey Cohen, to be "incredibly committed" to the project.
"He was incredibly generous with his time, not just about his character, but the whole movie. He came by the editing room and worked with us to help us perfect the film."
Ruben also said Sean did as many takes as he needed until he got it right and fleshed out his character.
"He came up with so much stuff that wasn't in the film, like the way he brought Mickey to life - and one of my favourite lines in the movie, where he goes 'Here comes Santie Claus'. That was just something he came up with on set and to me it's one of the best lines in the movie," he added.
:: Gangster Squad is in cinemas now.