There are many remarkable things about the first Superman film, up to and including the obvious influence on every comic book movie that came later. There wasn’t much of a blueprint in pop culture for what a serious look at a comic book character should look like. There were not yet giants who had shoulders on which Superman could stand.

But what really made the film so special was the performance of the late Christopher Reeve, the only actor who could make the idea that no one recognized Clark Kent as Superman due to his glasses even remotely plausible. His performance as both Clark Kent and Superman kept the characters distinct, and it was done through his body. Christopher Reeve was his own best special effect.

One scene shows this transformation perfectly.

It happens after Superman takes Lois flying, right before her date with Clark Kent. He nearly tells her the truth, and shifts into the part of Superman to prove he is who he’s about to say he is.

The amazing part of this performance is how clearly you can see Christopher Reeve shift his body from Clark Kent to Superman. His voice changes a bit, sure, but it’s all there in the body language. It’s a powerful, physical performance that doesn’t require a change into the costume or any of the special effects that went into the flying scene. The burden is on Reeve to sell the transition, and holy hell does he do it convincingly.

He appears to grow about a foot taller, his neck lengthens, his shoulders square and he shifts his body weight forward to lead with his chest. His mouth, which is almost turtle-like as Clark, turns into a confident smirk as his jawline somehow seems to square.

The whole thing only takes a few seconds, but Reeve was able to convince the audience that Clark Kent and Superman were two very different people. Of course Lois Lane didn’t realize who he was; every aspect of his body changed when he shifted between these two people.

The transformation is just as interesting when he loses his resolve and has to quickly transition back to the “role” of Clark Kent.

Reeve trained under David Prowse — who provided the physical performance of Darth Vader although James Earl Jones provided the voice — and he was a student of the Alexander Technique, a way to learn how to change bad habits of movement in order to hold yourself with ease. His body, and how he used it, went a long way toward making Superman a believable film.

Superman tells Lois is height is 6 feet 4 inches during the interview scene, if you’re curious about how tall he’s supposed to be, or maybe how short Clark is supposed to be. I doubt Clark would give his own height at more than around 6 feet 1 inch, and Reeve is able to show that difference using just his body language.

The next scene is from the Richard Donner cut of the film, where Lois pretends to shoot Clark Kent to get him to admit he’s Superman. Reeve makes the transition from Clark Kent to Superman happen instantly, he drops Clark and turns into Superman as an unconscious part of his reaction to the gunshot. Lois recognizes him as Superman even before he removes the glasses.

Watching it play out silently is helpful, since it shows how well this transformation works as a purely physical performance.

It wasn’t that the characters in the movie couldn’t see past the glasses, it was the fact that audiences were able to believe they were two different people. That was something Reeve achieved, and no one else has been able to replicate.

If we believed it, we had no reason to think the characters would think any differently. The least realistic aspect of the movie worked perfectly, and it was all due to Christopher Reeve.