I can't say whether this is true for all writers, but I would imagine it is to varying degrees, especially in fiction.
The characters I created for Roller Derby Vampire Girl are somewhat representative of real people and real interactions I have had throughout my life. Most, if not all, of them, exaggerated and combined in ways to feel organic.
As the book progressed and the characters developed, they all grew into their own entities. Often going back and changing things as I realized, the character wouldn't have done or said that. I've spent ten years with these characters, so it is easy to get to know them.
Regina was inspired by a good friend of mine and a former co-worker at her core. This strong woman who was upfront and let her emotions sit at the surface because if she let people discover how she was feeling without her narration or clarification on those emotions, it always felt too vulnerable. This aversion to vulnerability I really try to explore in the novel.
A supervisor once described her as a rose. A beautiful flower, but with thorns. She quickly corrected them. She was a cactus. Just leave her alone, and she'll be fine. (We had a unique work environment.) But as anyone knows, cacti often have beautiful flowers themselves when tended to. That one interaction and that aspect was something that indeed became the heart of who Regina was. Regina wants people to think she is far more dangerous and self-sufficient than she really believes she is.
Veronica, Regina's best friend, has a nurturing and quiet quality that paired well with Regina's over-the-top personal theatrics. Again, based initially on another mutual co-worker and friend, her character has probably gone through the most intensive changes and growth.
In many early drafts, she was just the typical best friend. Nothing wrong with it, but nothing honestly of depth as she was initially little more than a sounding board for Regina's plans and ideas. It wasn't until I really decided to explore their friendship that I realized how strong of a character she was despite her often timid and meek exterior.
Tabitha's character has always been a witty and smart character who deserves better. No one ever really deserves terrible things happening to them, but Tabitha gets put through the wringer. However, I worked diligently in rewrites to not have her just be a victim. I can't change her circumstances, but I could change how she reacted to them. Many of the people who have read the book to this point tend to like her. Amongst many openly flawed character traits surrounding her, she is easily the most likable.
Bambi is a vampire. (no real spoiler there) She is cunning and smart. She matches Regina's blow for blow. I like her as a villain because I could have easily written the book from her side, and I think readers would have liked her. It's nice to have an antagonist you love to hate.