The actor plays one of several LAPD officers assigned to a special task force, with the aim of keeping gangsters - and specifically Sean Penn's Mickey Cohen - out of LA.
"My character's picked for the squad because he's been able to stay alive, he has good instincts and knows when to take cover," Gosling told Flicks and Bits.
"He's not trying to be a hero, he doesn't have any fantasies about that. He served his time overseas and I think he feels like he's done fighting.
"He's come home and there's this other war and he doesn't feel like it's particularly his war to fight."
"We thought that Gangster Squad would be a blast, but we had to be serious on this movie - which was very hard for us!" he said.
"Once you've been a knucklehead in front of somebody, then it's hard to pretend you're like Humphrey Bogart. She's great. We get a little goofy together so I think it was hard for us to be dramatic and be serious. But by hook or by crook, I think we kept a lid on it."
Gangster Squad will be released on January 10 in the UK, and January 11 in the US.
Watch a trailer below: