I'm not sure if it's actually him playing, but from what I've been able to dig up, they hired a hand double to do the hand shots, and then never actually used him because what Jensen did came out so well. So I think he was faking it? But faking it really really well. I think. Like, they taught him how to make it look right, and he did it really really well.

Although I've also heard that he can actually play piano, so I don't know? Maybe he can play and knew enough to effectively fake playing that?

either way, it's Jensen's hands playing piano and makes me happy. :D