Give me that silent movie music
When silent movies were the only game in town theaters employed pianists or organists to provide a commentary sound track. Here is an example although not accurate but you get the idea. The piano player sat in a "pit" and watched the screen and simply injected tunes or dramatic music to kinda "fill in the dead spots". He/She was expected to know all the pieces and the new stuff on the market and just bang it in.
I was living in Santa Monica 20 or so years back and "Beauty and the Beast" (the original) was playing and Phillip Glass wrote music to take the place of the piano player. A couple hours of Phillip Glass is a lot. He writes very intricate "rhythmic harmonies" - chords that are often broken up (arpegiated as opposed to all notes sounding at once like a church hymn) and the breaking up very subtly changes meter and therefore accelerates or decelerates and you aren't sure why (metric modulation).
The difference between the solo piano player in the first example and Mr. Glass's planned accompaniment is, of course, huge.. The lone player can do what he/she wants and stretch it out as the spirit moves. Glass has to be perfectly timed - in the Hollywood film score tradition. See?