I enjoy this part. I do have a folder with pictures of people I found on the web. It’s like getting to know a good friend. Once you have your primary characters, give them an A4 sheet of paper each. Write down everything about them. I have a questionnaire on their details.
Their name, appearance, personality, relationships, friends, motivations, the past, present, goals, significant events in their lives, where they live, their religion, culture, family, etc.
Just get to know them. You do need back stories of what happened to them before the time of your novel. I love doing that with the characters I create, and I get to know them very well.
These are the characters that are important to your story, but not as important as your primary characters. Write up about a paragraph for each of them. This is to give you an idea of their attitude, personality, back story and motives, as I also have photos of these people.
These are the characters that only appear once or twice and are never heard from again. Now, although they are tertiary characters, all of them are still significant, and you should stay in touch with what happened before the story. Note down all of their names on a piece of paper and their purpose, so you don’t forget them. Again, I keep photos and outlines of every character in Scrivener.