Jeremy Piven is expected to appear live and in person before an Actors' Equity grievance committee tomorrow in order to defend himself against producers of Speed-the-Plow who accuse him of faking mercury poisoning last December to escape his contractual obligation to the Broadway production. It was originally assumed that Piven's lawyers would plead his case, but the Posts's Michael Riedel says the hard-partying actor is expected to surface. Unless, as one producer quipped, he "has too much sushi on the plane from LA." Hey-oh! When he quit, Piven's physician claimed the mercury levels in the star's blood were "very elevated," and last month Piven agreed to a second blood test administered by a doctor hired by the producers. (The results of that test are not known.) If the grievance committee rules against Piven, he'll be forced to settle financially with the producers or risk expulsion from the stage union. And though there are five actors on the committee, the unnamed producer says that doesn't necessarily bode well for Piven: "I don't think many theater actors are in favor of what he's doing. They take jobs on Broadway very seriously, and he's a Hollywood actor, behaving like one."