Though, actually - it's pretty much always there when I'm writing. Just yesterday it was bloody persistent and took the form of a Newsnight Review panelist:
"...and that entire section of 2nd person, I mean - what on earth was that? Why bother? Oh yeah, observations about life, blah blah: meaningless. Nothing to do with the story, or the plot, just a contrived attempt to jazz up a - quite frankly - very dull novel."