In his book, Your Screenplay Sucks, William M. Akers admonishes us to make all our characters, even minor characters, memorable and fascinating in some way. He believes that no role is insignificant unless you make it so.
In Body Heat, for example the writer gives assistant D.A. Ted Danson, one of several minor characters, an interesting habit – he pretends he is Fred Astaire, often doing little dance steps as he swings across the room much to the bewilderment or amusement of others. It is a small thing but this makes his character interesting. He stays in our minds long after the film has ended.
In the film, Down Periscope, one of the the minor characters, Seaman 2nd Class ‘Sonar’ Lavacelli is blunt and a bit of a roughneck, but he is also loyal. He has amassed an impressive collection of whales sounds on tape. When, during the war game, he finds himself in the presence of an enemy ship, he plays the sounds of whales having sex in order to confuse and divert the attention of the enemy away from the submarine.
Eccentric behaviour often does wonders to beef up a minor character. Imagine a character who is so obsessed with order and neatness that he measures the distance of every object in his room after cleaning to ensure that it is in precisely the same position as before. This not only speaks volumes about his character, but it is visually interesting to watch.
Or someone who is so spotless that she washes and shines the vegetable cans she has purchased before placing them in the cupboard for storage.
You get the idea. Go have fun with the minor characters in your stories by giving them interesting and fascinating things to do.
Make each character colorful or unique in some way in order to make her memorable and fascinating.