The key moment that convinced Zack Snyder to cast Henry Cavill as Superman came in the actor's first screen test for Man of Steel. 

The director had previously revealed that he knew only the right actor would be able to wear the Superman costume from the Christopher Reeves' era in this day and age and still look the part. 

So he had Cavill wear it for his first screen test and the rest is history. Now Snyder has released an image of Cavill in costume from that screen test and we can see why he was convinced. 

Snyder shared the image to his Vero account along with the caption: "First test with Henry... I knew right away he was my Superman,"

The director recently spoke about the test to Dejour late last year: 

"He put it on and when he came out of the trailer, everyone was like, 'Dang, you’re Superman!'"