This one is actually from the world of classical music, but I think it's relevant to theatre as well.
"When you go around the country and play with mediocre orchestras, the rehearsal circumstances are such that it's usually professionalism that carries the day, rather than optimum circumstances. It can be very painful. When you're playing a concerto, for instance, you actually have to avoid listening to the orchestra; you have to sort of protect yourself in order to play...In very few cases do you have orchestral players actually listening. When they do, it's a treat, but usually they don't." - André-Michel Schub, quoted by Joseph Horowitz in The Ivory Trade: Music and the Business of Music at the Van Cliburn International Piano Competition.
Substitute "mediocre cast" for "mediocre orchestra" and you have summary of the dilemma faced from time to time by actors who find themselves playing opposite (or, worse yet, being directed by) people who are clearly out of their depth. As Mr. Schub notes, it can be very painful. Happily my own experiences in this regard have been relatively rare, but I've heard my share of War Stories form other actors that are a bit hair-raising, to say the least.