You have to write a good score that you feel good about. At least, you're supposed to. But, if the director hates it, it ain't going to be in the movie!
'Sinister' is the first score I've done in which there's no orchestra in it whatsoever. There are traditional instruments I sampled, then manipulated, so you don't even recognize the source anymore.
If it's fast, no I don't have enough piano technique. In that case, it's probably been done on some kind of synthesizer or sequencer. Then the score can then be printed out and so forth.