Keeping a steady beat! That's the most essential thing. No matter what happens, you have to stay in tempo. You can't slow down for the hard parts or go back and fix mistakes like you do at home.I once tried to do some rock songs with a drummer friend of mine, just piano and drums. I naturally assumed that he would lay down a steady beat that I could "riff" on (if I may use some up-to-date slang.) Not so, I was informed rather coldly. I was expected to keep a steady beat in the left hand, the bass line, for him to follow. I found out later that that's the job of the bass guitarist in a band -- keep going plunk plunk plunk while others get to improvise freely. If there's no bassist, the piano does it.
This actually also depends on the drummer, what he's used to do. Loads of drummers only seem to be able to drum their own beat and if they cant adapt to you, you definitely have to follow him.