One of the many audience participation lines one can yell in the long pause during The Rocky Horror Picture Show. Tim Curry sings the following line to Brad and Janet during his grand entrance:
So come up to the lab
and see what's on the slab
I see you shiver with antici
And during that long pause, there may be an eerie silence (otherwise unheard of at a RHPS), in which someone will finally yell, in an orgasmically throaty voice, "SAY IT!". Another well-known retort is this: "This movie would completely suck without audience partici--", where Tim Curry finishes the sentence for you.