As writers we all have different inspirations. What we see, do, hear and experience all contribute to the voices of the characters we create. That I know of, there are two types of character writers: those who listen to the voice of the characters and sit back to watch them evolve, and those who dictate the voice of their characters and are involved in every facet of their growth. The strangest thing is when you witness someone who is both of them at the same time.
I have been that someone.
When writing, sometimes I feel as though I am merely the tool through which this character is telling his or her story. And yet in the same piece there will be other characters whose thoughts, feelings and actions I rule over with an iron fist. These other characters can do nothing, think nothing, and say nothing that conflicts with the way the main character is growing. A character is a focus for telling a story, imparting a moral message, or even just relieving stress. Once on paper, however, they can take on a life of their own. Those characters become the most beloved. Harry Potter is a fantastic example of a character who has grown far beyond his creator, but he is far from the only one.
Creating a character is not unlike having children; they are noisy, unruly, and frequently frustrating, but they also bring joy, pride and a sense of accomplishment to us as writers. When creating a character, it's best to start with the basics. What does this character look like? How old is he or she (or it)? Where is it from? Why is this character important? Who is he or she? There are times these things are critical to a story and times they don't matter at all, but unless you know your character you can't adequately tell their story.
Some writers use character creation tools to help them define their characters. Others wing it and come up with masterpieces. I've found that having something similar to a Dungeons and Dragons character sheet helps me with character creation. For those unfamiliar, Dungeons and Dragons is a pen and paper role playing game that has existed since the 70's. It is very basic, but then again, that's the point. Identifying the basic characteristics that make up an individual character is kind of the point. It lets you figure out who they all are and why their stories matter, things you need to know yourself before you can commit them to writing. Something like this allows you to identify a general archetype for your character to build off of. Character archetypes are something universally understood; they're the basis upon which we build our understanding of ourselves and each other. They allow us to distill the insane complexity of an individual being into simple, easy to understand terms. The Hero. The Villain. The Sidekick. The Fool. The Fanatic. The Egoist. The Revolutionary. These are all examples of simple archetypes that we see in stories all the time and yet despite their prevalence we still find ways to make them new, unique, and invigorating.
We make them human. Even those characters who are demonstrably inhuman, like aliens from another world or anthropomorphic cars, characters have traits that remind us of ourselves and have a realism that gives them form. Writers make these characters empathetic by showing them to be more than just an archetype. The Fanatic can start out unsympathetic, until we learn her fanaticism came as a direct result of personal tragedy. The Egoist can start out despised until we learn his ego-centrism only exists because no one ever tore him down. Once that happens he is forced to rebuild himself, and that's where readers connect. It's why Disney chose Iron Man as the first film of the Marvel Cinematic Universe; because the hardest person for the audience to sympathize with would be Tony Stark. In the beginning of the movie, he's rich, he's egotistical, he's self-centered, and a merchant of death. He's made his fortune off of other people's pain and we are meant to despise him. By the time he reaches his ultimate growth in Avengers: Endgame he's still egotistical, still rich, still self-centered. But he's also changed on a fundamental level that lets viewers see him as more than he was before. He's become human.
There are no unimportant characters. Every character in a story has a purpose, no matter how simple that purpose may be. Once you have down who your character is, then you can move on to what his or her role is in your story.