He may be staring as a gang boss in new TV venture Peaky Blinders, but the actor has opened up about his softer side. Cillian has two sons with his wife Yvonne McGuinness and has become more emotional since becoming a parent.
"I watch nature programmes and find myself weeping. Fatherhood has definitely made me vulnerable in some ways," he told Time Out.
"But sex and violence is justified if it's not gratuitous or fetishist. Although my kids can't watch most of my stuff till they're 18. But I'm not bothered and they don't give a sh*t."
Peaky Blinders is a new six-part BBC gangster drama set in post-war Birmingham on the cusp of the 1920s. It's set to be a bloody and violent affair.
"Most of the violence is slow motion, which I hate. But [director] Otto Bathurst had a clear vision and he nailed it. I knew we weren't going for realism," Cillian said.
The Irish actor with piercing blue eyes is no stranger to Hollywood blockbusters
Starring in Inception, The Dark Knight trilogy and In Time over the last few years he still has big acting ambitions.
"I'd love to have a go at shouldering a big blockbuster. I'm 37 now and the drive and hunger to make good, exciting work is still there from when I was 20," he said.
And he's not shy about who he admires in the business.
Admitting it doesn't inconvenience him when a fan asks for a photo, he's proud to have avoided the trappings of fame.
"An A-lister like Matt Damon is the most defiantly normal guy and one of the most successful actors in the world," he gushed.
"I admire that, and if you do that relentlessly enough, the wrong sort of people get bored."