When you’re watching a movie, you can see the actors as they deliver their lines. You can see the expressions they make, and you can hear their tone of voice. Seeing them talk tells you a lot more about what they’re saying than just the words they’re using.
When you’re reading a text, you only have the words the writer gives you. The rest you have to make up yourself. As a writer, you have to take that into account.
In theory you could write down exactly everything that the character speaking does – every single motion, conscious or not. In practice, that would be nigh impossible, and it would almost certainly be incredibly dull.
