In my opinion, one way to create interesting and detailed characters is backgrounds, and especially important/tragic events that happened in their past before the plot of the book.
Especially those past events that make a character behave a certain way (It’s difficult for them to trust new people they meet, or it’s difficult for them to make new friends, or they are aggressive for no reason, or they drink a lot etc.) for a big part of your story, without explaining the why to the reader.
When this behaviour becomes finally explained to the reader by the revelation of the character’s background or past event that scarred him (maybe through flashback), it will be an Aha moment for the reader who will understand why this character behaved this way throughout the story.
Also when the character realises he has to accept his trauma (if you choose to go this way with his past) and move on with his life during a scene that contains a pivotal moment for the character (maybe near death situation), then the character grows and changes psychologically by having a character development moment, which makes him interesting, especially if he changes his behaviour after that moment (like he becomes more friendly and trusting of people, or quits drinking etc).
Also this character development moment is pivotal for the character because he realises he shouldn’t let his past shackle him psychologically anymore and decides to finally act upon something that limits him.
If a character’s parents let’s say lost their lives in an accident in the past, the character at the present will feel overwhelmed/depressed by the event if he haven’t dealt with the trauma yet. So it will be carhartic for him and the reader when he decides to deal with his trauma, because the reader will identify with him.
Also i believe if you write a character that is happy go lucky or is social, friendly and trusts people easily, and you show him interact with the rest of your characters this way for a long time in your story, when something tragic happens in his life during the duration of the story, if you manage to show the shift in his behaviour after the event, it will be interesting because the readers will see the change in his behaviour and his growth as a person when he overcomes the trauma.
I hope i helped even a little…