Director Sergio Leone worked with Clint Eastwood on the Dollars Trilogy, and with Robert De Niro on his gangster epic, Once Upon a Time in America. When he was asked to compare the two actors, this was his response:
"It’s difficult to compare Eastwood and De Niro. The first is a mask of wax. In reality, if you think about it, they don’t even belong to the same profession.
Robert De Niro throws himself into this or that role, putting on a personality the way someone else might put on his coat, naturally and with elegance, while Clint Eastwood throws himself into a suit of armor and lowers the visor with a rusty clang.
It’s exactly that lowered visor which composes his character. And that creaky clang it makes as it snaps down, dry as a martini in Harry’s Bar in Venice, is also his character. Look at him carefully. Eastwood moves like a sleepwalker between explosions and hails of bullets, and he is always the same—a block of marble.
Bobby, first of all, is an actor. Clint, first of all, is a star. Bobby suffers, Clint yawns."
Quote comes from an interview with Sergio Leone, by Pete Hamill. Published in American Film, June 1984