The temporary change in personality that occurs after being deeply involved with a movie, book, game, or some other work of fiction.
Can also be expanded to nonfiction ("documentary lag" or some other variant), but is less common because those works are typically experienced through the frame of the reader/player/watcher's default personality.
CHRIS: Don't think. Don't hesitate. Just say it.
ASHLEY: Why are you talking like that?
ASHLEY: You sound like a movie trailer. Is this because you just finished watching Fight Club?
CHRIS: Uhh... yeah I guess I'm a little fiction lagged.