Give Supporting Characters Time to Grow

Many authors like to come on strong with secondary characters. Like it’s an art they want to show they’ve mastered.

Yes, you should come on strong with all characters by creating memorable details, but don’t deliver an information dump that halts the story's pacing for a character that doesn't matter as much as the hero/heroine/villain. Just as with main character introductions, sprinkle don’t douse.

Renni Browne and Dave King explain, in their book "Self-Editing for Fiction Writers," that when authors introduce a character all at once, they are "stopping the story cold for an overview of [the] character." These include personality summaries, back-story or flashbacks, and full physical descriptions. Long intros are "plain obtrusive."

You also commit a big author sin when with long introductions: you break the reader from the story and make the author present. The reader can just feel the author's wheels spinning, typing out each line of unnecessary description.

Browne and King explain that when you deliver characters’ information in excess, "you risk defining them to the point that they’re boxed in by the characterization with no room to grow."

Instead, try to:
...introduce a new character with enough physical description for your readers to picture him or her... a few concrete, idiomatic details to jump-start your readers’ imaginations... But when it comes to your characters’ personalities, it’s much more engaging to have these emerge from character action, reaction, interior monologue, and dialogue than from description.

When character introductions are given too much time and attention, they also block plot, and since action is character, you inadvertently block the character from acting out their true persona, making them feeling unreal to the reader.

So give your secondary characters time to grow through scenes, through action, through dialogue, and move on with the story.