I wrote a short story set in the "near-future" where an inventor has developed tech where you can experience movies as the characters. He uses an husband and wife friends of his as beta testers. They can't agree on the movie so the inventor sets the tech to choose the movie at random. The inventor's idiot choice lands the husband and wife in the 1980 film The Blue Lagoon. And because the tech chose the roles at random, too, the husband finds himself as Elva Josephson/Brooke Shields's character.

they can't change the plot but they can move around a bit independently and change the dialog slightly. Yeah they become the characters - the Husband is rather ****** off that he's going to spend the next few years of relative time as a girl, as Brooke Shields, no less, running about a deserted island with no clothes on. Because it's in beta there's no way to stop the program or let the inventor know what movie they are trapped in.