The 44-year-old actor reprises his role as fugitive Counter Terrorist Agent Jack Bauer for the first time in four years in the series.
The previous eight seasons were shot in real time over a 24-hour day, but the number of episodes has been cut by half for this story and Kiefer insists the change works because it allows Jack to be thrust straight into the action.
Speaking at a screening of the first episode of the series at Old Billingsgate Market, he said: "This is the first season where 12 episodes will represent a 24-hour day, the episodes are all in real time just like the show has always been but there are gaps in time. For instance in Season Two Jack is on a plane coming back from Central America to the US, the flight was two hours and it was the most boring two hours of '24' we ever had because I was eating peanuts on a plane for two hours, I'm stuck in real time. The way to fix that was to crash the plane in the middle of the flight, because it just wasn't working. Now if I need to get somewhere, I can get on a train or a plane or a helicopter at the end of the episode. This format allows the writers to speed up the time to the next episode and carry on in real-time there."
Kiefer was joined at the screening and Q&A by his co-stars Yvonne Strahovski - who plays new character CIA field operative Kate Morgan - and Mary Lynn Rajskub, who is reprising her role as systems analyst Chloe O'Brian.
The action star believes the real-time format of the series has always been helpful to him as an actor but is "problematic" for the director and writer.
He said: "The format and the nature of the show I think is inherently more difficult for the director and the writers. The truth is everything is in real time so for the actors it's actually very easy to know where you're at. "There isn't a missing gap where you have to figure out what your character might have gone through or might have done in those blank spots. I've always found the format is very helpful for an actor and problematic for a director."
'24: Live Another Day' starts tonight (07.05.14) at 9pm on Sky 1 HD and Sky 1 +1.