Characters are funny things. In many ways they are completely open. In other ways, they are very closed off. Knowing when they are supposed to be one or the other to the reader, is a difficult lesson to learn. Primarily because everything depends on circumstance.
Most writers botch this art thoroughly in the form of thoughts: when the writer goes into detail about a character's thought process during a scene. It's the one time we really get into the character's head and see exactly what they are thinking. But often, what most writer's get wrong, is that few stories are told from the prospective of a character looking back on their life. Most characters are living in the moment.
One of my biggest pet peeves is "information dumps." Most characters understand themselves and should not feel the need to explain themselves to themselves. In Shakespearean dramas, such things are called an "aside." If you need to say something that helps the reader to make more sense of the character's life, then you have done something horribly wrong in previous scenes.
The ultimate tool for a writer is CONTEXT and HINTING. Nothing is ever stated bluntly in a novel because most characters are too engrossed in their dilemma to be thinking clearly. Aside from that, most of them don't constantly feel the need to reiterate their backstory as they go through life. This would be a little weird if someone in real life went about thinking such things like:
As Marty walked through the halls of Lincoln High School, he considered his upbringing at the family cabin by the lake. He had two sisters, Lizzy and Beth, twins actually. His dad was a pastor at the local church.
NO NO NO. It's not relevant to what the character is experiencing and it just sounds awkward. It's the author creating an information dump. It's especially easy to do when working on a series where the lives of the same characters are followed. When this happens, it's tempting to practically summarize what happened in the previous book at the beginning of the new book. Like I said... Most characters live in the moment and do not have their life story rolling through their mind 24/7.
Now... how do we fix this?
I told you before that Context and Hinting are powerful tools. Let's look at Marty once again. If we NEED to know that he grew up in a lake cabin, we can use things in his life to help the reader catch on to that bit of information instead of blatantly stating it.
To do this, we can start a new scene with Marty sitting at home and interacting with his family. His dad might ask how he likes his new school, to which Marty might answer that he misses being home schooled back at the lake house. Then, his sisters might pipe in. Marty might smile at the way they always speak at the same time, a twin trait, he guesses. The twins might say they don't miss the lake house at all and that they like their new teacher.
Same information gets across, but it sounds less awkward and it's not an information dump. The reader still understands the same ideas about his past. It's all about CONTEXT and HINTING. Never just dump information into a story. Characters should think and process things the same way anyone in real life does.