Follow the lead of the soloist when interpreting rhythm and tempo. If she wants to increase the speed, it is your job to follow her. Work with the soloist ahead of time to nail down a precise tempo. Usually the accompanist will start the piece, so it is imperative that you get the opening tempo correct.
Do not overpower the soloist's playing. When accompanying a woodwind player you will have to lower the lid of the piano to diminish the sound. Brass players will need the lid raised completely and vocalists will need the lid somewhere in the middle.
Practice the accompaniment until you can play it perfectly from memory. This will help support the soloist and ensure that you are providing the best possible accompaniment.
Listen carefully to how the soloist plays, interpret rhythms and articulates notes. As the keyboardist you should be able to match his style.