Neil Patrick Harris gave the symbolic finger to an audience member who interrupted his performance in Hedwig and the Angry Inch with an "I love you, Neil!" Harris reportedly yelled back, "I'm doing something up here, motherf–ker!"
The actor's representative put a positive spin on it, telling Gossip Cop: "The response to the fan yelling was actually done in character--Neil as Hedwig, not as though Neil stopped the show to respond out of character."
When stage actors talk about "breaking the fourth wall" they mean communicating with the audience in a self-conscious or out-of-character way. It's a technique that can be used to great effect and has been for centuries. Shakespeare and his cohorts wrote plenty of "asides" into their scripts. But it's another thing when audience members break the wall, defy the code that says they're not to make a sound during a theatrical performance unless it's to applaud or cheer (or boo).