It's fun going back to watch Irvin Kershner's 1980 sci-fi epic "The Empire Strikes Back," knowing what twists would come in subsequent sequels. In "Empire," Princess Leia (Carrie Fisher) became angry at Han Solo (Harrison Ford), a man Leia knew was attracted to her. To make Han jealous, Leia brazenly grabs a nearby Luke Skywalker (Mark Hamill) and kisses him full on the mouth. Luke knows that Leia was only playing a game, but was happy to be kissed. Han stormed out of the room, outraged. Luke smiled in smug satisfaction. 

In Richard Marquand's 1983 follow-up film "Return of the Jedi," it would be revealed that Luke and Leia were siblings. This was clearly a last-minute plot twist, meant to parallel the "surprise" from "Empire" that the villainous Darth Vader was actually Luke Skywalker's father. The "Jedi" twist wasn't thought out, however, as it retroactively incorporated into "Star Wars" an...