No props, no fun.
Director George Lucas made his first Star Wars cameo in the movie Revenge of the Sith. He was a Pantoran senator, Baron Papanoida. You may have not recognised him all in blue. His daughter also cameoed with him in the same scene.
Cheaper than extras
Both R2 and C3PO became so popular that they had their own animated television series for a short period of time and even made an appearance in episodes of The Muppets and Sesame Street.