Iron Man
Over time, Robert Downey Jr. was allowed to ditch the full Iron Man suit while filming. As it turns out, the Iron Man suit is "really, really uncomfortable and pinchy," according to animation director Marc Chu. In the later Iron Man movies, the man in the suit isn't actually RDJ. It is either a CGI rendering or a stuntman.
Avengers: Infinity War
Even though Thanos was one-hundred percent CGI, Josh Brolin's acting was able to be relayed on the screen down to the smallest details. A process called Masquerade scanned Brolin's face, studying the minutiae of his expressions. Kelly Port, the VFX supervisor, said that she was most proud of capturing the "subtleties of [Brolin's] performance."