spaz, the class James Franco is teaching at NYU is very limited in enrollment (10-12 graduate students). I am sure they have to cap it even for majors. So the chance of you sitting in would be non-existent (IMO).
I guess if you are really a fan and want to time it so you can be near the classroom where he teaches when the class ends, you may get a chance to spot him. Though I am not sure if the school will tighten security for his classroom. (Just don't everyone get the same idea, otherwise it will be a nuisance for the poor man.)