It is not a very long part, but outstanding.  He plays Lt. Col. Bill Kilgore, commander of the first battalion of the 9th Cavalry Regiment, 1st Cavalry Division, who is eccentric to say the least.  Kilgore's world revolves around surfing!  

The attack of the 1/9 against the Viet Cong village to the tune of Wagner blaring is a scene that will be in the top memorable scenes from Hollywood for all time.  

Great cameo, well, slightly more than a cameo, in this ambitious film about Vietnam that was based on Conrad's novella "Heart of Darkness." Humorous, but Duvall's character really seems like a Marine, it's one of those brief roles where the actor, as trite as this sounds, makes you suspend any lingering notions of disbelief through an evocative and engaging performance.

Like all truly gifted actors, Duvall simply becomes the part - always managing to erase any line between his own personality and the character. And never did he more completely accomplish this than he did in this unforgettable role. He gave such a tight and realistic performance here that I completely believed he was the unfeeling, self absorbed ass that he portrayed. He not only took an almost cartoonish character, fleshed him out and gave him dimension but also (if youve been in or around the military) made him frighteningly recognizable. And continuing in his tradition of doing a lot with very little, he also summed up the jerks entire being in one of the most memorable sentences ever spoken on film, "I love the smell of napalm in the morning". It wasn't just the words; it was the dreamy look on his face - as though he were remembering his first crush or the smell of homemade bread. Absolutely chilling and more frightening than all the old film monsters combined.

Seems to me that this movie is far overrated.