The actor, who plays Tommy Shelby in the BBC Two gangster drama set in 1920s Birmingham, has revealed the latest series will be a departure from the previous two.
In an interview with Digital Spy, he said: "There's no point in just replicating what you've done before - you have to push it. But there were times where I was like... f**king hell."
While remaining tight-lipped about the specific details of the new series, Cillian continued: "What we're doing with Tommy now is a massive departure - certainly from series one, and from two - but that has to happen. Four or five years have passed in the characters' lives since we first met them, so you have to push it.
"But what [series creator] Steve Knight did was just take the opposite tack and go east, to get involved with Russia. That gives it a much more interesting flavour - it's much f**king darker and weirder and more interesting," he added.
The first episode of the new series, sees a mysterious visitor turn up on Thomas Shelby's wedding day, who will put the entire Shelby family in harm’s way.
Paddy Considine has also joined the cast as a rogue priest, who has been described as "the definition of evil".
Watch a preview of the new series in the video below...
Peaky Blinders airs tonight (5 May) at 9pm on BBC Two.
Tap the image below to launch the slideshow...