A song’s structure makes it more difficult to ad-lib than dialogue. Nevertheless, the eight cast members in Musical! the Musical are able to improvise a tuneful 90-minute production night after night on the basis of audience suggestions–including a three-note combination that will provide the foundation for one of the score’s highlights. They cover all the bases in this improvised Broadway show (formerly at the Royal George): romantic ballads, inspirational anthems, full-cast dance sequences, an intricately harmonized spectacle just before intermission, and a tear-jerking finale with the obligatory a cappella penultimate chorus. As with all good improv, however, what keeps the parody (of the film The Sixth Sense on the night I attended) from degenerating into juvenile silliness is the wholesale commitment of the cast (including Michael Descotaux, the lightning-fingered accompanist) to their hastily assembled universe. It might be a comment on the state of musical theater nowadays that even lines like “Every suit of armor has a few dents / Every beating heart has a sixth sense” can make our throats tighten and eyes moisten. Theatre Building, 1225 W. Belmont, Chicago, 773-327-5252. Through October 8: Fridays, 8 PM; Saturdays, 7 and 9:30 PM; Sundays, 2 PM. $24. –Mary Shen Barnidge