Unlike a medium shot of a shield or a hammer, when Tony Stark is building his arc reactor in the cave scene, there is an extreme close up shot of him soldering the wiring.
More than just creating a prop here, Russell Bobbitt had to teach Robert Downey Jr. and, in a way, help the actor become Tony Stark.
So Robert Downey Jr. needed to be taught how wiring works so that he could perform that scene and come off believably.
It’s harder to fake it with an extreme close up where the camera can see every detail.