I think I've mentioned it before, but it is worth mentioning again that my preferred style of writing is multi-POV (point of view). While I don't get as hardcore as George R. R. Martin, in my series Urban Legend, I've got three distinct POVs going. Each subsequent installment will have three POVs, but switching out a few so by the end of Rylee's first school year at Artemis Academy there will have been six POV characters with varied levels of alliance and antagonism (not mutually exclusive in at least a couple cases). Writing each POV character requires me to fully flesh out those characters as I would any protagonist. While not all writing styles match mine, I think it is very important for writers to consider minor characters as assets and not afterthoughts.

From Uni-dimensional...
During my brief but not insignificant foray into screenwriting, I was taught to think about the major characters as "round" characters and minor characters as "flat" characters. The reason being, in screenplays there is limited real estate on the page so the focus needs to be the characters that matter. In the years since developing my writing, I can say with absolutely conviction that this unequivocally false; every character needs to be as "round" as the next. Viewers and readers have become more sophisticated in recent decades and uni-dimensional characters aren't going to cut it. The talking heads, linear love-interests, and the sycophant sidekicks now spark irritation and outrage as demand for realism is at an all time high. Dragons and cyborgs are fine, but the princess better be more than a "basic DiD" (Damsel in Distress). Even if the reader never actually sees the full complexity of a character, like the tavern owner in two pages with six lines of dialogue, they need to feel as if he was a real person, otherwise the whole sequence will be indeed "flat."

The “Plot-point” Character
One of the most pointless characters is the one who serves a single plot driving purpose and then becomes irrelevant. This is the "talking heads" whose sole purpose to to drive the Exposition Express right down the readers gullet, full steam ahead. Or the deus ex machina character who was maybe hanging around like loose garbage than at the end helps save the day in some small way. And of course there is the comic relief: the funny person who, at best, offers a few quips; or at worse, hinders the protagonist in contrived comedy to pad the conflict (nobody likes that guy). These kinds of characters pop up in all genres, and if poorly done enough can throw the reader out of the story with caricatures and not characters. A litmus test to see if such a character plays such a role, pluck them out entirely and see how intact the story remains. If the plot only needs a few minor plot holes to fill, you might want to consider dropping them OR perhaps spend some time around upping their significance by making them multi-dimensional.

The “Character-point” Character
These serve a bit more useful purpose, if nonetheless still limited in function. Character point characters tend to evoke or amplify certain emotions or traits in the protagonist to help them on their way. The love interest might only be there to be pretty and co-dependent (both guys and gals) and ultimately serve no ground-breaking plot-purpose. Or they could be that teacher who shows them the "Ways of the Whatever" and then either hangs around or drifts away because they got nothing else to do. Or, it could be that lovable scamp who ends up dying and drives the protagonist to go Super Saiyan or something and drives home the finale (or the next installment). While these roles have a bit more depth, they are still merely roles, a box the minor character is packaged in and shipped off. Instead of dropping these characters though, they sill have the framework to be potentially integral characters. To lift themselves off the page as a simple "LOL" to a "BFF".