The British beauty plays the 'Argo' filmmaker's right-hand woman in the gambling flick, and was shocked when Ben threw in some surprise moves on the film set.
She said: ''We had this weird back story and a relationship in the film years ago, and he has this possessive thing over me.
''My character is very dependent and cutting. In one scene Ben's character finds out that I've just had a night of intimacy with Justin's character and he gets very jealous and pounces on me - that wasn't in the script. It's fun but he really sprung it on me.
''There's a lot of that, he's a director as well so it was cool. I love all that stuff. I'm like, 'Bring it on, do something that's gonna freak me out!' What was on the page became much more than what was on the page, much richer!''
The former Bond Girl also says she didn't really identify with her character, Rebecca Shafran, in the new action movie because she still isn't comfortable playing a femme fatale.
Gemma added to OK! magazine: ''I wish I was like Rebecca in real life because she's so bold, but I'm not. I'm too giggly. I say something and I'm like 'not really' ''! She's much more deliberate than I am.''
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