Interviewer: Sometimes what doesn’t work in terms of the translation from script to screen is exactly what you were talking about – an actor’s physiology itself can alter what a film is about.
Murch: Yes, that’s a good point… [in Apocalypse Now] the character of Willard is a re-actor. He doesn’t do anything in the film as written. He is the observer, and you watch the film through his eyes and sensibility. And it was clear to Francis after a month of shooting that [Harvey Keitel] was just not the right chemistry for that role. Marty Sheen has very big eyes, and Harvey Keitel has thin eyes, and so it’s easier its easier to use Marty’s face as the lens through which you see this war than Harvey’s… There’s a lovely aphorism by Bresson which says, “The little gleam of light caught in the actor’s eye gives meaning to his whole character.” [A] little chance spark of light in an actor’s eye tells you what the character is thinking, or makes you think you know what he’s thinking. And the chances are with an actor who has big eyes that you’ll get more of those reflections rather than somebody whose eyes are in deep shadow or whose eyes are thin. It’s a very true observation, but its one that is completely uncommented on.
via Walter Murch on Editing and His Translations of Curzio Malaparte | Filmmaker Magazine. (Emphasis mine.)