You know what to play by listening. Some numbers will already be busy, backing-wise, so not much may be needed. There's a time to play a 'wash' over some, as in a gentle chord palette on a strings type sound under everything else. You may, in a more punchy number, match the bass line with a synth sound, to beef things up.Listen to how brass stabs work in a lot of recordings. Just a stab here and there in usually more upbeat numbers gives something no other instrument (except synth!!) can, apart from maybe drums, and you can work arrangements with the drummer to this effect.
Some numbers need the same sound all through, others will benefit from several sound changing line by line, of verse/middle eight. Again, listen carefully to recordings, and catch a particular sound that could only occur a couple of times, but that's all that the song needed.
Be prepared to do a solo using sounds that no-one else can provide - a flutey type in a ballad, a saw-tooth sound in a raunchy number.You'll have a good dozen sounds at your fingertips, straight on call. Don't be afraid to experiment - even the bandleader will be amazed when you pull something out of the bag that he didn't expect, but it fitted.
And don't forget - you don't have to be making noises every second of every number ! Watch horn players in a band, silence works well. Unless you're a drummer or bassist... You're probably going to change your volume up and down more than other band members, so a volume pedal is a great asset, freeing up both hands.