''Lead character'' and ''character'' in books and films

Dear Everyone,

I've realized there's a confusion in my head regarding with what expressions you refer to

1) the main characters in a film,
2) the main characters in a book,
3) characters in a film,
4) characters in a book.

I only know there're ones which are rather used referring to either films or books, and perhaps there're ones used for both.

Please, could you let me know the terms one can use for the above? Thank you!