One way I like to go about filling out characters is to interview real people. If your character is a teacher, interview a local teacher. If you are writing about a bartender, go to a pub (during off hours), tip well, and ask for their story. Not everyone wants to be interviewed, but enough will that you can find someone. Ask them about their daily routine. Why do they prepare in the ways they do? What are the problems that are obvious to someone in that profession that aren’t clear to an outsider? You can get some great background information for your characters this way.
Your goal should be to create a character who is believable in their profession to both laypeople and experts. You want the computer programmer in your crime story to feel authentic to your grandmother and to the software engineers at Google. However, one pitfall of this engorgement of character information is putting too much of it into your story. It’s useful to know how the waiter in your story serves their customers and the shorthand they use for taking orders. But does that level of detail need to be visible in every story? No. Much of the information you come up with on your characters will serve to bring them into focus in your mind, and as a result they will be clearer to the reader. Think of all those small details as being marked ‘for your eyes only’. Your reader won’t know what they are, but they’ll know if those details are absent.