Well, it turns out that I was partially correct in the two actors assumption. Partially. There were two main techniques the crew used to shrink Cap. down to his original, scrawny size. The first one of the two (the one I was closest to guessing) indeed required the use of a double. Leander Deeny played Evans' double before Steve Rogers goes through the physical transformation that allows him to become the super soldier that stands for all that is American. What they did to cover for the fact that the two actors clearly don't have identical faces, was to place green dots all over Deeny's face to use as motion trackers. Using these motion trackers, animators were able to take Evans' face and place it on Deeny's body. This was achieved by having Deeny act out each and every shot pretty much exactly the same as Evans. Then came the hard part.
That was the easier of the two techniques. The other one (and certainly the more time consuming one) required an entire physical makeover. Basically, the visual effects specialists would take the shots with Evans and literally reshape the majority of his body, making him smaller and thinner. This includes everything from thinning out his neck to completely reshaping his jaw line. The reason for sometimes using this technique over the other one is...well, I'm not quite sure. Apparently the motion tracking was simply less effective in certain circumstances. I don't know what those circumstances must have been, but I'll leave that up to the professionals. For a more detailed explanation of the entire process, take a look at this behind-the-scenes video:
Glad that another mystery in my life has been solved. Onto the next one!
