I have a habit of creating characters to fill a hole, then realizing that they're useful for a several other scenes. Several of my favorite characters have begun simply as "I need someone to carry this message to the main character" or "Well, I suppose someone needs to man that station." Paying attention to these people pays off big when you need something down the road and the best to make sure they're useful when the time comes is details.
So your MC is going to the library to check out a book on car repair. Why they're not looking online is irrelevant, maybe it's a story from the fifties? Point is, at some point in your jaunt to the library, they're likely to need assistance from or at least bump into the librarian. What is that character like? Now, in your mind, they probably just exist to show your character where the book is and feed them an ever so important bit of narrative that leads to your character discovering their hand-me-down car is actually worth millions because it belonged to a famous gangster once.
As you continue in the story, you'll likely find that that librarian can come in hand again. If you've fleshed them out, given them motivations, desires, wants, and needs, then they can crop up naturally throughout the story. Perhaps it turns out your librarian is a 1920s gangster buff and the description your MC gives of their car jogs the librarian's memory. That's great for the plot. But if you want to make people believe in your librarian and, maybe even get attached, how about they chew on pencils when they think? Or maybe always bring an orange in their lunch? It's these little details that breath more life into the characters that otherwise would've been just a flash in the pan.
For me, characters like Katie, Samantha, Giz, and Simon all have little quirks that don't necessarily advance the plot, but they keep the characters feeling real. Here's one to keep an eye out for in when the novel is released. One of my characters loves Boo Berry. Is it critical to the plot? Not really, but it's a little touch that makes them feel more human.
Be Excellent to Each Other.
Keep your eyes open for my debut novel, The Paladin.