In my writing so far, I have though much more about plot than characters, which I think is not necessarily the best way to go. I have the main character and have thought in detail about what happens to her and some supporting characters, but I think it'll be boring without at least one other main character. So, I invented a character who is a recently-retired detective. He is not really necessary for the plot, but I thought he could reveal information to the reader through his investigation and also help her in the end. He can gather much more information than a regular person such as my main character. I thought of giving him a bigger role though. I thought of a subplot involving him that is related to the main plot. He sees a link between what is happening with the MC and his first case, which was a murder that was never solved. I'm wondering if I'm doing too much here? I don't want the reader to be so confused they stop reading. Have you ever come up with characters just for specific purposes even if they're really negligible in the results or events of the story just to have someone else? I don't want him to be meaningless, which is why I came up with this subplot. How do you come up with your characters and the roles they have to play?
You're talking about retired secondary detective characters. For some reason, I'm suddenly thinking of Frost. He's a character that many can relate to and putting a character like Frost in secondary position, I imagine, will make readers wonder what your protagonists are like. But is he there to help the mc or otherwise?
I don't think you should be thinking about the characters and the story as separate things; the characters ARE the story. If you don't have any meaningful interaction between characters, then your story is likely to be rather thin, unless your story is very short or you're a very good writer. I suppose that explains how I come up with characters, too - they're usually generated by the story itself. You might find that if you delve a bit more deeply into some of the 'supporting' characters and their relationship with the protagonist, you've already got characters ready to become main characters. Of course this is all guesswork without knowing what your story or characters actually ARE.
