In 1980 Alberca et al. described a patient with a syndrome of increased muscle irritability, who presented ondulating muscle rolling movements and electrically silent cramps, myoedema and muscle reactions to mechanical stimulation similar to myotonic response, suggesting a disfunction at myofibrillar level.
For c-3po, it was a costume, Anthony Daniels played c-3po: For R2-D2, It was a costume and a remote controlled robot. CGI was also used in the prequel trilogy. So, they had Kenny Baker to play him : And from Portrayal section of wookieepedia article on R2-D2 :
Excluding The Clone Wars (the movie and both animated series), here's the actors who were in both trilogies (but not always in all films): Anthony Daniels played C-3PO and Kenny Baker played R2-D2 in all six movies. Frank Oz, as you have already mentioned, played Yoda's voice in five movies. Ian McDiarmid played Senator/Chancellor/Emperor Palpatine (and Darth Sidious) in 4 movies (Episodes I-III and VI) originally, but was dubbed in for Episode V later. So you could say he was in four of the movies when they were originally released, but is currently in 5 of the movies.