Yvonne Strahovski has to get her hair cut every three weeks for her role in '24: Live Another Day'.

The blonde beauty portrays new character CIA field operative Kate Morgan who is handed the task of apprehending fugitive former Counter Terrorism Agent Jack Bauer - played by Kiefer Sutherland - in the action drama.

Yvonne has revealed the show's unique 24-hour, real time format presents unique problems, such as maintaining the same style and appearance so each episode is consistent.

Speaking at a screening of the first of episode of the series at Old Billingsgate Market in London on Tuesday night (06.05.14), she said: ''(During shooting) don't put on weight because it's really bad for the show and we're all cutting our hair every three weeks or four weeks. We have to otherwise it would grow and then it doesn't match.''

The series has been off air for four years since Season Eight aired in 2010 and lead actor Kiefer, 47, admits he was pleased to get four months notice to get in shape for the show's numerous action scenes and daredevil stunts.

The actor - who joined Yvonne, Mary Lynn Rajskub and several other castmates at the screening and Q&A - said: ''The show is really physical and my experience with '24' is that it's not a question of if you get injured, it's a question of when and how severely. So the better shape that you're in, the better chance you have of getting through a shoot without getting into real trouble. I was lucky that I had four months to prepare for this, I wasn't doing another show.''

The cast are still shooting the final instalments of '24: Live Another Day' in London, and the series starts tonight (07.05.14) at 9pm on Sky 1 HD and Sky 1 +1.