Hopefully as a relief teacher you'll be given enough warning in advance so you'll know what language points you need to prepare. Doesn't always happen though. I don't see why you can't use the student course book but some teachers prefer to cover major grammar points themselves and the relief teachers sometimes do "everyday" language and speaking activities.
This greatly varies and depends on how long the class is. Most relief teachers are given some sort of indication of what needs to be done - the class teacher should do this and if it doesn't happen then the school is probably a bit slapdash. When I've relieved, I've always tried to compile a series of supplementary material I can bring along and use (at various levels). You're new to teaching so this will take a while to build up. ESL websites can be a useful source of materials. I'd probably focus on a particular langauge point like vocabulary, do some things in the book and then do related exercises from the workbooks (it there are any) with any backup material you have.