If this is with a friend or in public, do an "Epic Nothing", where you freeze on the spot and do nothing. It could be midway during conversation or something. The thing is that it builds up an awkward atmosphere and people expect you to do something. Try to hold for as long as you can, then either freak them out or resume as normal, just to gauge their reaction.
You could wait until they are asleep then wake them up screaming and yelling fire, shaving cream trick , shave there eyebrows off , shave there head bald , butter the floor , mouse trap on alarm , ice cold water , serene rap on the toilet , serene rap there car or a doorway so they can crash into it face first I have more , but I don't think they could all fit .(x