Bring back a throwaway character.
You had a character who’s sole purpose was to impact some tiny bit of information or do an action. The throwaway character has an endearing trait but would never be seen again in the story. Make the character a reoccurring character. Take another throwaway character and combine the two. Give the throwaway character a job that makes the character able to appear again and again in the story. Think about the guy with the lisp behind the hotel check-in counter in the Eddy Murphy movie.
Make the minor character frustrating because they live in their own world and don’t care about the hero’s world but hold key knowledge that the hero needs. Have the throwaway character only give the knowledge up if asked the right question. The gardener knows that the dead lady was allergic to raspberries because he grows prize winning raspberries and she wouldn’t let him grow them at her place. Or the busybody gossip saw the yoga instructor in Prada high heels at noon time when the instructor only wear tennis shoes and has been seen bare footed in public.
Don’t forget the villain’s sidekick. Write about him doing the villain’s dirty work. Building don’t magically blow themselves up, somebody has to plant the explosives and it takes a lot of explosives to blow up a building. Show the villain’s sidekick planning to shove the villain aside and take over. Better yet have the sidekick plan his escape (he’s smarter than a rabbit and a rabbit always has more than one exit hole out of it’s den). Still better yet show the sidekicks has his own agenda, he’s going to take the explosives that the villain plans to use to blow up the massively polluting power station and use them to blow up the National Security Administration’s satellite data gathering center.
Remember throwaway characters, minor characters are important. Some throwaway characters have spawned their own wildly successful series.
Write on, draw on. Professor Voltage