There may be pictures to guide your answers. If you really don't know how to say something, try to think how you could say the same thing using expressions you do know, for example:
You want to say "I‘ve lost my bag" (J'ai perdu mon sac), but you can‘t remember the word for "lost". Why not say:
You may have to answer a surprise question. You will not be given any information about it, or the required response.
During your preparation time, try to guess what could be asked according to the setting of the role-play.