I tend to sprinkle short, humorous vignettes throughout my stories to amuse the reader, even when those bits don’t always push the plot forward. Some writers maintain that every word in the story should advance the plot in some way, but I don’t always agree. The Harry Potter books were chock full of these not-important-to-the-plot but fun-to-read-about bits, which is one of the reasons I enjoyed the series so much. The trick, however, is figuring out how to slip these bits into a story without causing problems.
For example, let’s say one of my minor characters is an alchemist who turns out to be allergic to many of the ingredients he uses to make his potions. It’s not important to my plot (although I suppose I could always try to arrange things so it was), but I decide the amusement factor is sufficient to add it in to the story. My first inclination would be to reveal this fact through some exposition, perhaps via a quick one-liner if possible, but I know I’m supposed to show, not tell, so my second option is to either reveal it through dialogue or by writing a scene where he experiences an allergic reaction.
But here’s where it gets tricky. If I devote too much time to delivering this information via a scene, my critique partners (and ultimately my readers) will assume it’s important to the plot. And when it never gets mentioned it again, they complain. Loudly.
And it’s not just humorous bits either. Spend too much time on any aspect of your story—even descriptions—and you’ll have your readers scratching their heads. I recall reading a story once where the author spent half a page describing the way in which a road meandered down a hill, along with a description of secondary crossroads, the surrounding terrain, and compass directions. He gave so much information, I braced myself for the battle I assumed was about to take place there. Turns out that location was never mentioned again.
Sometimes you want to trick the reader into thinking some irrelevant fact is important, especially if there’s a mystery involved. But if that’s not your goal, you should always keep in mind that the reader subconsciously depends upon the cues you provide to decide what’s important. Don’t accidentally lead them astray.