In Walk The Line, June Carter calls out Johnny Cash's reckless behavior by saying "Y'all gonna blow this tour. You can't walk no line." Cut to Cash in the studio recording "I Walk the Line." Presumably, there's a deleted scene where Carter tells him to avoid getting the Folsom Prison Blues from a Boy Named Sue.
In Notorious, Lil' Kim very conspicuously refers to Biggie as "Big Poppa" during sex. Later, when she asks, "What do you like about me?" he replies, "I love it when you call me Big Poppa," a line from one of his biggest hits. This might as well have been followed by a wink to the camera and a Mentos pop.
In Jersey Boys, the Four Seasons are watching some old movie where a man slaps a woman, to which one of them says "I bet you she cries." Another responds, "Big girls don't cry ..." Cue instant jump-cut to their fully fleshed out hit. The song literally starts the moment his line is finished, like they were terrified the audience would take a bathroom break and forget this scene.