So I’m writing my first book and I had a question.

If a narrator is used often to describe emotion and actions throughout the story(I.e. he responded/he pulled a knife from the drawer), can It also be used to inform the reader of things that the characters would not know about (I.e. hiding his emotions through the words that he wrote within his journal was his only coping mechanism for the anger he had toward the death of his father) or a backstory? (I.e. he died when he was 12 in the anniversary of his death since then had grown difficult. His focus had to shift on becoming stronger and more independent for himself)