Everyone knows that in a novel there is at least a main character (MC) and their antagonist - the person who makes the MC's struggle real - and secondary characters, their friends or sidekicks.
One set of characters some novice writers forget to add judiciously are the background characters - the extras in a movie. These are the people on the street, in the restaurants, or neighbors. These people help flesh out the MC.
Since I can use my books without anyone's permission, I'll use them for examples. In Kathryn's Beach there is a convent. The most prominent nuns are Sister Elizabeth and her sidekick Sister Theresa. However, other nuns walk in and out of scenes, perhaps say a line or two, and leave. It wouldn't be very believable that Saint Mark's was a real convent with only two nuns. To tell the truth, I had to keep a list to remember which nun was the school principal. They weren't that important to me or to the story to remember their names. But they played an important role in making the convent and the two main nuns seem more real.
The same goes for the mention of the traffic on the freeways in Los Angeles. I don't make a big deal of it, but it would be lacking to not mention the traffic problem that caused the detour Kathryn and Sister Theresa took that lead to Sister Theresa's big question to Kathryn in High Tide. Wouldn't it seem odd to drive all over LA and never mention traffic?
Also at the airport in High Tide, even though Kathryn couldn't go to the gate now like she did, she could when the ms was written (pre-September 11, 2001). It makes sense for the traveler to bump Kathryn with their carry-on bag and the people she encounters on the escalator. LAX is a very busy airport. There had to be people in it for anyone to believe Kathryn was at LAX. Whereas, Kansas City International (KCI) closes down early and no one is around when you get off the plane - very spooky!
Storm Surge switches settings requiring a whole new group of background characters had to be cast, like the gardner. Some of them had been introduced in High Tide, so the trilogy carried over from one book to the next. Otherwise it would be like Kathryn had been plucked out of the two previous books and plunked down into a different story all together.
Keep in mind camera angles and you won't forget the background characters. There is always a wide angle or pan to show the other people in the location, then it narrows onto the principle characters per that scene. Pay attention to that when you watch TV and translate it to your writing - don't forget to cast a few extras in your novel.