Well, one way to do it is to have roles that have some power triggered by things said in the thread. Could be somthing simple, like "You can't be nightkilled on odd nights if you say "Wondertwin powers activate!" at least once during the day" to more complicated things like "If someone agrees to be your friend during the day, then X happens" or "You have to get 3 other people to say "Candyman!" during the course of a day in order to use your night power". This works best in games with a lot of flavor, encourages posting, and also the wierd looking behavior will give the town something to talk about. (Also note that "powers activated in thread" don't have to be pro-town powers; although scum powers activated that way are harder to balance, the possibility of them encourages rampant paranoia, which is always a good thing.
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