Filling the twentysomething, gross-out, low-budget movie void is first-time writer-director Rob McKittrick's $3 million comedy about a gang of ''Shenanigan's'' chain-restaurant waiters (Ryan Reynolds, Anna Faris, Justin Long) who do wacky things like cough up loogies into mashed potatoes. ''It's very light on plot. I really just wanted to get across the feeling of what it's like to be in that world,'' says McKittrick, who was a 23-year-old waiter when he wrote the script (and was waiting tables at a central Florida T.G.I. Friday's when an agent asked to represent him). ''It's a smart dumb movie, but yes, there are more d--- jokes than anyone has a right to.''