The actress Zoe Saldana is in both of the movies. She plays Gamora in the Avengers and Guardians of the Galaxy movies and plays Neytiri in the Avatar movies.
There will be four (or more) Avatar sequels. If James Cameron is able to capture even half of the interest for sequels as the original Avatar, then Zoe Saldana will likely become the actor involved in the highest cumulative total box office. She is already ranked tenth.