Keep a character list that includes each person’s description, habits, motivation, and any other details that bring him or her to life. Refer to it as needed for consistency as well as new ideas for involvement in the story.
Especially when you’re starting a new story with characters who are just coming to life, it’s handy to have a reminder what you’ve done with them so far. Every characters should have a distinctive role in the story. If they’re redundant, then you don’t need him or her. Developing them can have other benefits as well, such as introducing new plot twists, or in the case of mysteries, red herrings.
Sometimes my favorite character in a story, either my own or someone else’s, isn’t the protagonist, but one of their supporting characters. Set yours up so they’d qualify for a virtual Academy Award for their performance in your novel.