In our most basic instruction on fiction writing we were taught to keep the story moving forward. A codicil to that principle is to maintain the reader's interest on every page. This is usually done by introducing new developments in the plot—an unexpected twist for the reader, or a major character's reaction to encountering something unexpected. These are good rules that should be followed. But in these comments I will argue for the effectiveness of another means of gratifying the reader.
I first noticed it in an old sci-fi novel by Robert Heinlein. He advanced the plot as he should, of course, but in the middle of the action he made a passing reference to a famous zoologist named Dr. Tiergarten. I found myself laughing because Tiergarten is the German word for zoo. The effect was momentary, but it definitely gratified me as a reader.
These momentary comic effects in the midst of drama were standard in the classic movies. No one would question the increasing tension in the movie Casablanca. But as it builds, the C.Z. Sakall character turns around and suddenly bumps into a character we already know as a pickpocket. Sakall's hurriedly checking his pockets provides a moment of hilarity in the midst of the growing tension. In the Western My Darling Clementine, as tension builds toward the climactic gunfight at the OK Corral, the Ward Bond character whinnies like a horse at the beautiful Linda Darnell as she carries a washtub of water past him. She responds by dousing him with the water. Neither of these incidents adds to development of the plot, but both gratify the audience with momentary laughter in the midst of the tension.
I've tried to make discreet use of this technique a standard element of my fiction, though I tend to keep mine understated in the manner of Heinlein. The hero of The Lazarus File finds himself in a corrupt town run by a thoroughly corrupt sheriff, and in need of escaping town before members of a drug ring can capture him. During the escape he sees a billboard that flaunts the town's corruption by advertising exotic dancers from Germany, one of whom is named Kirsten Keinekleider. (The German keine kleider means "no clothing.") The effect is momentary, yet several readers have remembered it and reminded me of it.
One scene in that novel involves an elaborate hoax perpetrated on one of the villains. The hoax takes place in the Red Herring Bar, and the leading temptress tells the villain she comes from a village named Mirage which, she says, is very close to where they are sitting. Similarly, an incident in my Preston Barclay mysteries has two people mention a rumor that an incompetent psychologist thinks the hippocampus is a zoo. Other incidents of the same kind appear here and there.
I've used that same technique in my current novel, Lightning on a Quiet Night.
Needless to say, this kind of thing can be overdone. But, used judiciously, an author can gratify his reader in unexpected ways without detracting from the forward movement of the plot.