The first person shooter (FPS) sequence took about 14 days to shoot.
The locker the BFG is stored is designated IDKFA. This is the in-game cheat code that will give the player all keys, full weapons, and ammo.
Dwayne Johnson was offered the lead role of John Grimm but chose the role of Sarge because he thought it was more interesting.
(at around 1h 35 mins) During the end credits, when the cast members' names appear on the screen, the only ones not to be shot/blown up are the only ones who survive in the movie.