A straight line is an innocent sentence delivered by the straight man (le faire-valoir). For example, the straight man may say, "I have nothing to say," and the comic might say, "I know, but how will you phrase it this time?" Any thoughts?
Que penseriez-vous de "phrases d'accroches" ou "d'amorces". If I understood correctly, we are referring to the lines the straight-man says in order for the main comedian to respond and make jokes.
Sorry about the brief delay in answering, Engtrans26. Your two suggestions look credible to me because you are correct in assuming the lines are meant to give the comic an opening to make his joke or point. Are your suggested phrases current or are they your best estimates of what could be a translation of "straight line"? That is, would anyone recognize them?