The producer and director had to make sure their pick for the Man of Steel looked the part - and insisted on putting him in the much-maligned suit Reeve's Superman wore.
Snyder tells Entertainment Weekly magazine, "If you can put on that suit and pull it off, that's an awesome achievement."
The Tudors star Cavill admits he was worried about what he'd look like in the replica outfit - because he'd lost all the muscle he'd gained for mythical action movie Immortals.
He tells the publication, "All I could think was, 'Oh, God, they're going to look at me and go: He's not Superman. Not a chance.'
"The actor inside me was going, 'You're not ready! You're not ready!'"
But 300 director Snyder was suitably impressed: "He walked out and no one laughed. Other actors put that suit on and it's a joke, even if they're great actors. Henry put it on, and he exuded this kind of crazy-calm confidence that just made me go, 'Wow. OK, this is Superman'."