Just look blankly at them whenever they ask you a question and then start saying things like ummmmmm errr for a couple of minutes before asking them to repeat the question again.
Turn up with your clothes on back to front and tell them you are trying to start a new trend. (Maybe wear your pants on the outside)
Maybe ask them "Will this take long" and then half way through "are we nearly finished yet?" reason: I'm registered for a poker tourney that starts in an hour (obviously you won't be but they don't know that).
Sigh loudly every few minutes.
Ask if you can use the toilet half way through the interview and then take ages. When you come back say sorry had a heavy one last night or I must have eaten something dodgy (that will give them a nice mental image)