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The London-born actress, who plays Cathy Muller in the action-packed thriller and stars alongside Chris Pine, chatted to HELLO! Online at the Leicester Square premiere on Monday night.
"I think the major reason why I wanted to do this was for Ken Branagh who's one of my heroes," said Keira. "I used to watch Much Ado About Nothing so often when I was a kid that I actually broke the tape twice so my mum had to buy me the video twice. Working with him was a complete dream come true, and with Chris — what a lovely man."
The Hollywood beauty, 28, added that a change in film genre was just what she wanted after starring in the likes of Anna Karenina and A Dangerous Method.
"I got to the end of Anna Karenina and I realised that for the previous five years I'd either been dying or something horrendous had been happening to me," said Keira. "I think I just really fancied doing something fun and a pure piece of entertainment. This is the absolute epitome of Hollywood action and thriller which I fancied doing.
"I have two other films, one called Laggies and one called Can A Song Save Your Life? which are just unbelievably positive and lovely and fun and nice. I was desperate to do a bit of that after crying for so long."
Sir Kenneth Branagh, Keira and Chris Pine
Keira, who married musician James Righton last year in the south of France, attended the London premiere with Chris and Kenneth. Opting for a peplum black shift dress, Keira looked the picture of elegance as she had her hair swept into a chignon and wore a dash of red lipstick.
"Keira's fanatastic," said Chris, who posed next to the English rose for photos. "She's a great actress and beautiful obviously, and she brought a humanity to the relationship that my character and hers had."