The obvious fun that the cast is having works its way into the viewers, to the point where every moment feels like a party. This is one of those times that they turned chicken shit into nice pizza salad. It doesn't have to make sense, because, you know, it's pizza.
See it if you must, knowing most of the film and deaths are covered in the trailer. It's always kind of a bummer when a film gets hamstrung by its advertising, but never having seen the trailer probably wouldn't have made the film any more enjoyable.