Here is what the Oscar-winning director had to say.
"Sigourney and I have a long creative history, dating back to 1985 when we made Alien. We're good friends who've always worked well together, so it just feels right that she's coming back for the Avatar sequels. Her character of Grace Augustine, as fans know, died in the first movie, so she's playing a different and in many ways more challenging character in the upcoming films. We're both looking forward to this new creative challenge, the latest chapter in our long and continuing collaboration."
Although details surrounding her new character are being kept secret, some fans speculate that her death in the original Avatar was more of a transition into the world of the Na'vi. The currently untitled films will be shot back-to-back with the first being released in December 2016. The other sequels will be hit theaters 2017 and 2018.