Susie sees Jack playing Jazz at a club. He seems to be really into it, sweaty and shaking his head.
Then later she says to him "I saw you the other night playing in that club.....you're a fake."
I didn't quite understand what she was accusing him of. Was she essentially saying he was moved by the music more than he leads on, and so he's aways pretending to be a jaded cool guy ? I'm not really sure on this.
Yes, she was saying his jaded, cool demeanor did not extend to the music. He pretends to distance himself from everything, but he was in the music.
Susie is basically saying that Jack's cynicism is an excuse for inaction, or lack of involvement, but that Jack does not really believe cynicism is appropriate in regard to everything. She literally saw him cast that aside when he was performing the music he really loves. Why does that not apply to loving a woman?
And I think she also called him a "coward" somewhere in that conversation. I guess the reason behind that, besides being afraid to commit to a woman, would also be that he never was brave enough to pursue a music career in music that he truly loved.