He might have been bi-polar. Many, many great actors are. But you can still be stupid and be crazy. If you can ever find it, Truman Capote’s essay “The Duke in his Domain” is a textbook on Brando and his stupidity. And I write as a real fan of Brando.
Just as an addition - Steyn neglects so many of his wonderful performances: Julius Caesar, Burn, The Fugitive Kind, Reflections in a Golden Eye, Last Tango, The Nightcomers, Apocalypse Now, Godfather. I mean, it’s a long list!
"Last Tango" was when I really noticed that he was just phoning it in, but the critics were still in worship mode.