Movie Body Transformations

No I’m not talking about a normal guy turning into the hulk with poor CG effects. What I’m talking about has mainly to do with Spider-Man.
How did they make the main character look really skinny and wimpy one day (in the mirror in his bedroom) and then pretty buff the next?

Surely that must be near impossible to do with CG as it is his bare body, or is it?


Movie scenes aren’t shot in order, you know.