One problem that has always hindered the adaptation of books to film, TV or animation is that you have no way of conveying an inner monologue or characters' thoughts. Very occasionally a monologue is shown as a voiceover but this is considered mainly a cliché of the noir genre, or is only used at the start of the film to set the scene (and the end as bookends). Generally though characters either have to vocalise their thoughts, which often sounds unrealistic, or entire scenes that rely on a character's train of thought are cut.
What if artistic decisions had been made differently in the early days of the film industry and it became common accepted practice in (Western) filmmaking for voiceovers (or even text subtitles perhaps) to represent characters' internal thoughts?