Many shows also include subtle allusions to an actor's past career. "The O.C." has referred to a character who threw up a lot in "The Sixth Sense," a role played by "O.C." star Mischa Barton. And the Sandy character, played by Peter Gallagher, starred in his college production of "Grease." Gallagher starred in the Broadway version.
John Schneider played Bo Duke in "The Dukes of Hazzard" and now plays Clark's father, Jonathan Kent, on "Smallville." In "Smallville's" first season, Jonathan listened to the "Dukes" theme in his car.
"You sort of believe that if Bo Duke grew up, he could be this role," Gough said. "John brings all the good sort of icon baggage to the role of Jonathan Kent that you'd want. He's sort of Bo Duke grown up and now raising a kid who also bends the law in his own way."
And the jokes work on their own level: Audiences who get them instantly feel more hip, but those who don't are none the wiser.
"We try to build the joke so if you get it, it's a little extra tickle for you," Schwartz said. "And if you don't, it hopefully will still make sense in the context of the show."
"It's kind of like the old Warner Brothers cartoons," Gough said. "I loved them as a kid for totally different reasons than I love them as an adult. As a kid, a lot of those references just flew over my head."
But Schwartz admits he's not so sure how far to go with such references. When do inside jokes stop being fun and start taking the audience out of a scene?
"I'm sure there's a fine line, but we're happy to cross it," he said. "That's the only way to find out where the line is."