It makes sense.  The movie is headlined by three proven comedy commodities in Vince Vaughn, Ben Stiller, and Jonah Hill, along with secret weapon Richard Ayoade.  This gag reel ensures that moviegoers know how much fun all four actors had riffing off one another and promises lots of improvisational shenanigans.  The actual bits of footage, meanwhile, incorporate the movie's science-fiction elements and add a bit of slickness to the proceedings.

Akiva Schaffer, the director of The Watch, shared this combo trailer-gag reel through The Lonely Island.  Schaffer is, of course, a member of the comedy trio, alongside Andy Samberg and Jorma Taccone, and you probably know them well from their Digital Shorts on Saturday Night Live or their two albums.  This is his second feature film as director, following 2007's Hot Rod. 

He provides a little intro here explaining the thinking behind the trailer:

For a split second in there, you can actually see Billy Crudup, whose symmetrical face hasn't really shown up in the marketing.  He's not the only one, either, as past trailers, TV spots, and the like have focused on the core group of four and provided only quick bits from the likes of Rosemarie DeWitt, Will Forte, R. Lee Ermey, and Nicholas Braun.  Also included in this trailer is a quick look at an alien played by the great Doug Jones, covered by pounds of special makeup effects, as is his custom.

Check out IAR's press conference coverage on the movie by clicking here.

The Watch, written by Jared Stern, Seth Rogen, and Evan Goldberg, arrives in theaters tomorrow, July 27th.