McQueen?
Granted, I think Martin Sheen gets way, way too little credit for his performance as Capt Willard--he is, after all, the only character in every scene given that the story is from his pov, but I also think Brando fat nearly destroys the film.
Especially as he's presented near to the end after so much build up.
I recall reading that Steve McQueen missed being cast as Willard through some differences in salary, travel, etc, that were ultimately resolved, but too late.
So I'm left to wonder, if McQueen had been cast, would his name above the title been enough to allow Coppola to minimize or even replace Brando?
I believe, as I posted elsewhere, that a non-existent Kurtz in the finale would have been preferable to a fat Kurtz. There is simply no justification for it. It takes one out of the story completely & reminds the viewer that Brando's in the film to ensure its box office.
Of course it's impossible now to imagine any other Willard than Sheen, & I highly doubt McQueen would have gone to the extremes Sheen did, especially in the opening scenes. But, who knows, he might have brought his own style & somehow made Willard his own.
Mainly, I wonder if he would have been enough to offset Brando.