Ditto that. There were many very great movies in which he played integral roles that many don't relize was him.
Ned Pepper in True Grit, Frank Burns in MASH etc. He was in Apocalypse Now for maybe 10 minutes but it is his performace that everybody remembers.
Many folk don't know that Duvall also once played Dr. Watson in the 7 Percent Solution (A Sherlock Holmes story) If you didn't know it was him, you wouldn't have guessed it. A Great Actor indeed!