Fourth Wall

In acting lingo, the fourth wall is the imaginary wall that separates the actors on stage from the audience. They can't interact with the people behind the wall--doing so would break the suspension of disbelief the audience needs to really get into things. The same holds true for a MUSHing environment.

Characters don't (and shouldn't) know that they're fictitious entities. For example, if John Doe ran into James T. Kirk, he wouldn't recognize him as a character from Star Trek--unless Star Trek (as a television series) exists in his source's canon. Characters shouldn't reference pop culture items unless it would make sense for them to do so. Some characters might be canonically aware of their true nature, but they're few and far between, and most people think they're not quite right in the head.