Here's something that happens to a lot of fiction writers. We come up with a great premise and a bit of a plot, but then we realize that plots and premises are fine, but it's the people who make the story move forward. Even when we have a good idea of what the main characters might be like, we have the supporting cast and a whole host of "bit parts." To help get a good idea of what these characters are like, write a help-wanted announcement. Here's one for my work in progress which takes place in an underground settlement on the moon that looks a lot like a small town.
Wanted: proprietor of a general store on the moon. Must be conversant with the odds and ends of daily life. From everyday China to hammers, nails, screwdrivers, screws, and umbrellas, your job is to keep people supplied with what they need. You should also be able to deliver a little folksy wisdom. If people underestimate you or consider you eccentric, all the better. Some side work as a spy may be included in your duties.
A follow-up to this exercise would be to write out a series of interview questions for different "applicants" for the job. What would they list as their qualifications, prior jobs, hobbies, skills, etc? It can be a different and enjoyable way to create a backstory.