It’s called deep point of view. It means crawling inside your character and looking at the world through him/her.
The best way I know to do this is to use the word ‘I’ when you are writing because that forces you into your character’s mind. That’s called first person viewpoint, though what anything is called is irrelevant, the end result is all that matters. Using ‘I’ works especially well if you are writing a diary entry because diaries are written that way.
The thing you must remember, however, when using this point of view is that you can’t write anything into the diary that the character in your story wouldn’t actually know at that point. And if you try to make your character too all-knowing in order to get across some story point or other, it will come across as wrong to the reader.
So do it but don’t over-do it.