The Setup
Fresh off of The Godfather Part 2 and The Conversation, Francis Ford Coppola attempts to make an adaptation of Heart of Darkness, set against the backdrop of the Vietnam War. He chooses to shoot in the Philippines and plans on a four-month shoot. After offering the lead role to Steve McQueen and Al Pacino, who both passed, Nicholson, Redford, and Caan were all offered parts of either Kurtz or Willard. When they passed, Coppola's first choice for the role of Captain Willard is Martin Sheen (who'd originally auditioned for the role of Michael Corleone in The Godfather) but he'd been attached to another project, so they cast Harvey Keitel in the role, based on his work in Mean Streets.
The Execution
Coppola shot for a few weeks with Keitel but didn't like the way that Keitel was playing the part. He felt that Willard's role should be more of a passive onlooker, and Keitel had not been playing it that way. (I believe on Inside the Actor's Studio, Keitel mentioned that he and Coppola had friction over this point because Keitel had been in the Marine Corps and felt the part should be played a certain way). Differences in the film would have been the scene where Keitel steals Duvall's surfboard and brains him with it. Also different, the scene where he bash's Kurtz's nose with the butt of his gun then begins cutting off his fingers starting with the little one. He was let go, and Sheen came aboard to take the part of Willard.
What Might Have Been
Fresh off of The Godfather Part 2 and The Conversation, Francis Ford Coppola attempts to make an adaptation of Heart of Darkness, set against the backdrop of the Vietnam War. He chooses to shoot in the Philippines and plans on a four-month shoot. After offering the lead role to Steve McQueen and Al Pacino, who both passed, Nicholson, Redford, and Caan were all offered parts of either Kurtz or Willard. When they passed, Coppola's first choice for the role of Captain Willard is Martin Sheen (who'd originally auditioned for the role of Michael Corleone in The Godfather) but he'd been attached to another project, so they cast Harvey Keitel in the role, based on his work in Mean Streets.
The Execution
Coppola shot for a few weeks with Keitel but didn't like the way that Keitel was playing the part. He felt that Willard's role should be more of a passive onlooker, and Keitel had not been playing it that way. (I believe on Inside the Actor's Studio, Keitel mentioned that he and Coppola had friction over this point because Keitel had been in the Marine Corps and felt the part should be played a certain way). Differences in the film would have been the scene where Keitel steals Duvall's surfboard and brains him with it. Also different, the scene where he bash's Kurtz's nose with the butt of his gun then begins cutting off his fingers starting with the little one. He was let go, and Sheen came aboard to take the part of Willard.
What Might Have Been
The change of one Willard to the next didn't really do much, as the film itself is really the star. Sheen's stock didn't soar with the release of the film in terms of catapulting him to leading man status, and he continued to work throughout the years with memorable turns in the films The Dead Zone, Wall Street, and The American President, before putting in a performance as the President on The West Wing. In a twist, Keitel's career is probably elevated by not continuing as Willard. Instead of having to spend a whole year shooting in the Philippines. (Filming started in March of '76 and concluded in May of '77, and wasn't released to theaters until August of '79) he is able to shoot and release: The Duellists, Blue Collar, and Fingers, all great films which help advance his career.
So, let's say that Keitel doesn't get fired. Instead, Stallone, deciding he doesn't want to continue the Rocky franchise, branches out and gets the lead role in Fingers. This precludes him from writing, directing, and starring in Paradise Alley, and the world never gets to hear Terry Funk as Frankie the Thumper tells Lee Canalito: SHUT YOUR HOLE PUKE. That would have been a tragedy.