On the surface, Nick & Norah's Infinite Playlist is about young love. But behind, under and all around that love story, the movie is about the power of music and the manifestation of that power that is the homemade mix.

Sony Pictures

Kat Dennings and Michael Cera in a scene from Nick & Norah's Infinite Playlist

As the movie's title reflects, that manifestation has gone through technology-driven reformatting; what used to be a mix tape is now a mix disc or even a digital playlist. But while the packaging has changed and the songs vary, the mix remains an intimate expression of self.

Actors Michael Cera (Nick) and Kat Dennings (Norah) were in town recently, lounging through a day of interviews, and we took a break from movie talk to compare notes on the art of making the perfect mix.

Mr. Cera started. "For long drives and stuff," he says, "the pacing, the ride, the roller-coaster of it – that's what's important, what you have to pay attention to. If you go into a low point, you've got to put something in next to pull you out of it."

Ms. Dennings quickly established her master-class level of mixology by talking about the little details – cartoon sound effects, air-traffic-controller instructions, movie and TV show snippets, etc. – that mix-makers weave into their collections.

"I like to put skits in between songs on mine."

"I do, too," Mr. Cera says. "If you have the Monkees' album Head, there's a bunch of weird tracks on it, like, 'I'll have a glass of cold gravy with a hair in it, please.' And I like to put that kind of stuff on it. That's a nice bridge to your next song for a mood change."

And because of its power of seduction, the mix has inspired all kinds of rules – always open big and snappy, never put one sad song after another, and so on.

Ms. Dennings kind of wrinkles her nose at all this. "I don't have that whole 'Oh, this song should lead into this song,' and 'Oh, the intro to this song would be so good following this song' thing going on when I make a mix."

"I do sometimes," Mr. Cera says, unnecessarily, because you just knew he would.