Itís often said an underdeveloped character is a 2-dimensional character. A 2-dimensional character has no goals, no back story, no anything. S/heís just there as a prop, like the barkeeper your hero/ine meets for three seconds before never seeing him again.
Writers recommend using 3-dimensional characters ó characters with depth; characters with goals, dreams, loved ones, flaws, history, likes, dislikes.
Three dimensions isnít enough. Humans donít exist in the third dimension. We exist in the fourth. Time.
People are generally dissatisfied with their driverís license photo (even if it looks just fine). A single snapshot, freezing you in time, is not a good representation of you, because you exist in the fourth dimension, not the third or the second. You arenít that still frame. You are a living, breathing human being. While you canít live in a picture, you can at least keep a few minutes or hours of your life in a video. Imagine if driverís licenses had animated images instead of static pictures, like in Harry Potter, or .gif images online.
Humans grow and change. Weíre always moving. Weíre all dynamic characters, no matter how little we think we change. Change is rarely big and open and grand. Itís often subtle. Our goals, dreams, loved ones, flaws, history, likes, and dislikes may define a snapshot of us, but they influence us so much.
Most of the good ď3DĒ characters in literature are actually 4D. They live. They grow. Their experiences change them. Theyíre never the same person in the end of the story as they were in the beginning.
If you can help it, never use a 2D character. There are nearly always opportunities in the narration to drop hints at a personís depth. Even that bartender whoís only in the story for three measly words can be defined beyond ďbartender.Ē It takes almost no effort to, at the very least, give him something a bartender doesnít usually have ó say, a picture of his family painted on the wall behind him. Or he could have a cat purring on his shoulder.
What if, failing everything else, he just smiled a lot? Bartenders in fiction rarely smile unless itís in a mischievous or sinister way. Make your side-characters people, too (but donít let them steal the show).
If youíre not pressed for words, or if you feel you can do it in very few words anyway, go ahead and make your bartender 4D. Give him a sense of existing beyond your story. Let him react to what your hero/ine does and says with all the emotional ups and downs of a real person.
Needless to say, your hero/ine should always be 4D. If s/heís not, change that.
Writing prompt: take a character who was previously 2D and make them 3D. If you have no 2D characters, make a 3D character 4D.