Gerard Butler has admitted he channels his inner "madness" for certain acting roles.

The 'Copshop' star - who plays professional hitman Bob Viddick in the upcoming action thriller - has opened up on how he saw himself in both that character and his part in 2018's 'The Vanishing'.

Speaking to, he laughed: "I wouldn't call it talent, but it's definitely natural.

"I definitely have that - you know, that movie I made, 'The Vanishing', in Scotland a couple of years ago. It was interesting, delving into every part of that madness - I thought, 'S***, this is all me!'

"I have all this in me, I have light, I have a lot of darkness. So with Viddick, it was the same idea, tapping into that crazy part of you, that part that could kill somebody, or could be unleashed."

However, the 51-year-old actor also pointed to the importance of "having fun" on screen, even when it comes to the darker roles.

He added: "And then also having fun with it - that moment when I scream... You climb into the mind of a killer or a hitman, and try to take that as seriously and truthfully as possibly.

"But then also you go, how can I make this fun, and shocking, and interesting, and different colours and layers?"

On the subject of keeping things fun, Butler also joked he'd be able to step into a 'Mamma Mia' sequel in place of Meryl Streep.

He teased: "Yeah, maybe 'Mamma Mia' would be a good one! [laughs] I don't know if I can still sing, I gotta work on my voice cords again, get them back in order."

He joked: "I do think I could probably do a better job than Meryl in 'Mamma Mia.' "