Secondary characters - interesting people? Received wisdom suggests that there should not be too many and that they should know their place - in the supporting cast. I know quite a lot of authors have tales of the supporting character who tried to take over the book and had to be restrained. Of course, this can be a good thing, as sometimes they warrant their own book - and then hey, you have a series!
I've been thinking about this, as I've just finished writing a novella - the Christmas one, if you are interested, and I found one of the supporting characters taking on a very strong personality - not enough to dominate, but it made me happy whenever she was on stage, and a happy author writes better books. I like to think so anyway. That happened in both my published novels too - I have a soft spot for Suzanne, the heroine's mother in Never Coming Home and for Jonathan, the heroine's colleague and best friend in Out of Sight Out of Mind. The so far unpublished rom/com (Working title 'I don't know but probably something with Riviera in it.') which, fingers crossed, will be out this summer, has a best friend that I rather liked too. And that one has a bit of a story, which is kind of a reverse of taking over, as I have written a second in series for that - it's the one with the nut-case plot that might never see the light of day. When I came to consider a hero for the second in series I realised that I had one all ready in one of the supporting cast, but neither I nor he had expected that he would suddenly get centre stage and a book of his own,
Which just goes to show - you don't have to impress the author the first time around to end up with your own book. 😉