The 4th Wall. "The Artist is Present" Marina Abramovic

http://www.moma.org/interactives/exhibitions/2010/marinaabramovic/marina_perf.html

"This is not a theatre. Theatre you repeat. Theatre you play somebody else. Theatre is a black box.Performance is real. In the theatre you can cut with a knife and there is blood. The knife is not real and the blood is not real.In performance, the knife and the blood and the body of the performer is real. Performance is a unique way of art. Its temporary, it comes and goes. 

I like them to be present. The work is done for the audience. Without the audience the work doesn't exist. The work doesn't have any meaning."
- Marina Abramovic 

I like how she says right at the end that without the audience, the work is basically a waste of time. "it doesn't exist...doesn't have any meaning" 
Being able to break the 4th wall, allows the audience an opportunity to realise just how important they are in a performance.