Kiefer Sutherland spent six months battling nerves about playing Jack Bauer in 24: Live Another Day after a four-year break from the show.

The actor returns to his iconic role for a new season, in which the counter-terrorism agent is in London racing to stop an assassination attempt on the U.S. President, played by William Devane.

Bauer has been in hiding since the last series in 2010, and Sutherland admits he struggled to put himself in the agent's troubled mind space.

He tells Britain's The Sun, "Jack Bauer is harder and angrier than he's ever been... In the first few days we did a lot of running and action stuff and I actually felt really good about it.

"The most difficult thing in the six months leading up to shooting was dealing with my nerves and realising we're opening this up again and trying not to be sacred of it. I had to view this as a real opportunity to try and make the best 12 episodes."

The show is set to air in the U.S. in May (14).