The trivia items below may give away important plot points.
While shooting one of the fire scenes for the movie, a special effect fire got out of control and burned the whole set down, including the studio stage it was built into and almost all of the crew's equipment. Nobody was seriously injured and filming resumed the very next day with equipment being hauled in from all over the world.
As Doug (Bradley Cooper) pulls hornets out of his right ear with a cotton swab, Doug is reflected by three mirrors, effectively showing him four times from all angles (and the hornets as well).
With one exception, all the shots with Renée Zellweger being close to and around fire are real (practical effects). The one CG-enhanced shot is the wide angle of her sitting passively on the couch while the room was burning around her. This was shot twice: Once with her and no fire and once with fire and no Renée.