The fourth wall is wonderful. It protects performers, gives them the freedom to commit to their art. The audience benefits too, enjoying the show through an exciting, voyeuristic lens.
And that's great.
Except the fourth wall isn't real. So it depends entirely on human imagination and respect. Much like a child's imaginary friend.
When that respect disappears from either side of the wall, the wall comes crashing down.
Here are 6 of the best performer-audience clashes of all time.
1) Shake it. Stir it. Just turn it off.
Who'd win in a fight, Wolverine or James Bond? Another question: who could beat Wolverine and James Bond at the same time?
The answer is:
Not me, not you, and not the guy whose cell phone interrupted Hugh Jackman and Daniel Craig during a 2009 Broadway showing of A Steady Rain.
2) Transform yourself out of here
If an off-duty actor can't respect the invisible boundary, what chance do we have?
After an afternoon of heaving boozing, Shia LaBeouf made an impromptu trip to Broadway show Cabaret.
Shouting, smoking, and groping the leading man led to the Transformers star being booted out and thrown in jail.
3) Wingardium levi-oh shut up
Brit actor Ian Hart is best known as Professor Quirrell from those wizard-boy films. But he's also a burning ball of stage rage.
Irked by an audience member he thought had been speaking, Hart shouted at the man to shut up, before lunging into the crowd to confront him after the curtain call.
4) Orchestrate that thing off
You know what doesn't improve Haydn's Piano Concerto in D Major? A ringing cell phone.
You know what brings it to an abrupt halt? A second ringing cell phone.
Christian Zacharias is ridiculously chill:
Don't answer. Let it ring.
5) Love Actually doesn't live here
You're Keira Knightley. You're best known for starring in Pirates of the Caribbean. And now you're débuting on Broadway.
What you don't need is some guy screaming a marriage proposal at you from the gallery, before lambasting you for taking too long to reply.
The man was escorted from the theatre, but not before tossing a bunch of flowers at the Love Actually actor.
6) Curiously casual health warning
The Apollo Theatre on London's Shaftesbury Avenue is old – it first opened to the public in 1901.
Some 112 years later, during a performance of The Curious Incident of the Dog in the Night-Time, chunks of falling masonry brought parts of the balcony crashing down.
The leading actor's super-British warning to those in the stalls?
Have you ever seen a performer flip out on the audience?