My local theatre group has just started rehearsals for this one, too...
I believe it is pretty similar to the movie, because the fiddler is a metaphor for intervention... wether divine or otherwise... whenever something happens to change the course of events in Anatevka, you see and hear the fiddler, I think he is a pretty integral part of the show, but doesn't have any lines...
If you have ever seen 'a midsummer night's dream' it is a similar character to Puck, except that the fiddler doesn't directly intervene. To loosely use film terminology, it is almost as if the fiddler is like non-digetic sound. Kind of like additional music, that while we can hear and see him he sort of isn't really there in a way... he kind of represents the happenings in Anatevka, and particularly the events that happen to Tevye and his family.
In our group, the fiddler has to be able to play violin, but i don't know if that is expected of you.
Have fun smile