Many character structure sheets can be found on the internet with things such as age/race/features. But personally, I like to create my own fact filesheets on my characters so as to include specific words structures such as;
Xander would say ‘for death's sake’ or ‘Holy Death!’
These phrases help to structure his character. I also add a paragraph or two on the character's backstoryso as to root them into the world I have created.
I also find having some notes on the personality of your character helps to guide you later on when your character has to get out of a difficult situation. It makes it easier to think like them and to strategies an escape route from their point of view.
Sometimes sketching your character can help as a visual aid to keep the mind focused when coming to characters personal appearance.
But I always find the best way to immerse myself into a character's mind is to do some basic interview exercises. (Like on my exclusive character interview blog). You pretend to be your character and have other people ask you questions. Give the answers as if you were your character and before for you know it you end up sinking into the role of the character, as an actor would do and there for you give your character what I like to call ‘essence’.