The 39-year-old actress stars as Sydney Prosser - who is male lead Irving Rosenfeld's (Christian Bale) lover - and she locks lips with the 23-year-old star's alter ego Rosalyn Rosenfeld during a tense scene in the forthcoming film, which the flame-haired beauty is keen to take credit for.
She told Us Weekly magazine: ''I came up with it. I was just like, 'What if there is a moment where she plants one on me?' Jen did it in a brilliant way. She killed it.''
Amy and Jennifer got on very well during filming and make many laughs on set.
She added: ''We had a really good time. There is this one scene in the bathroom where I came in without my heels. I'm 5' 4'' and she's about 5' 10'' and then I had my big hair and my orange skin, so I looked like an Oompa Loompa. She and I took a picture documenting the moment.''
However, Amy says her ''dark'' character was different to other roles she has played and helped her develop her acting skills.
She added: ''This was the darkest character I can remember playing.
It was really difficult at times, but it made me grow as an actor.''