A person who won't take sides in a controversy(noun)
WordNet. Princeton University. 2010.
Use "fence-sitter" in a sentence
"I think they express my approach to this, which is somewhat in the fence-sitter/"observer" category as opposed to "directing" anything or anybody - or playing a role."
"I agree with Angela, I want to be a fence-sitter, someone in the audience who gets to enjoy the show and critique both the actor and the director."
"He (his staff) would have written it, he would have buttonholed every g.d. fence-sitter and laid it out for them."