What sold Yoda was Frank Oz. Period. And the actors on Avatar were there in person, recording mo cap. Obviously their surroundings were green screen but they acted against each other, not a wooden poll. In LOTR, there also were actors and partial sets present, the rest enhanced with CGI. Andy Serkis was also on set, on location with the actors portraying the hobbit. For his more intimate scenes, it was just Andy Serkis by himself, playing off himself.