What can dialogue and internal thoughts often tell the reader?
They can give us insight into the character's personality and motivations.
Have you ever read Catcher in the Rye? Pretty good stuff, right? Take out the dialogue and internal narration, and you have the story of an unremarkable teenager walking around New York City doing unremarkable things. Most stories are like this.
If, instead, you have a man of action doing amazing stuff, you probably won't hear a lot of interior monologues, because that's not the focus. Conan the Barbarian and Beowulf aren't famous for their insightful observations or their witty ripostes. Choose your literature carefully.