I don't know when I first saw this scene, but I know that when I did, it was at a time when I knew next to nothing about movies, and even less about acting. I've learned a little since then...enough, at least, to be able to recognize that film acting is very much its own strange art, one which requires its practitioners to pore themselves--their emotions, their characters, their thoughts--into specific, stylized moments, moments captured in close-ups, lines of dialogue, and brief movements, all so as to give editors and directors the ability to build a story from the splices of film they shoot. On the stage, as a traveling thespian, doing summer stock and memorizing the classics, Hopper probably wouldn't have amounted to much. He wasn't a great actor in that sense. But when he wasn't completely wasted by drugs and alcohol, and when he was in the hands of a smart director (or directing himself), he knew how to take words, how to look while saying them, how to glare as they came out of his mouth...and the result, often, was cinematic gold. Those 30 seconds from Apocalypse Now taught me that, and have stayed with me for decades.
A great performance, in a great film. Rest in peace, Dennis.