The 71-year-old actress - who played Dr. Augustine in the first movie - is returning for the long-awaited sequel to James Cameron's sci-fi epic and revealed some of the challenges she was up against.
In an interview with T magazine, she said: "I had some concerns. But that's what the training was for. And I really wanted to do it. I didn't want anyone to think, 'Oh, she's old, she can't do this.'"
Sigourney trained by diving in Hawaii and Florida and she and the cast, including Kate Winslet, had to learn to hold their breath for six minutes underwater.
Once she was ready to go, the 'Alien' star had to be placed in a giant tank with weights around her waist to film.
The actress won't be returning as the same character in the sequels to the 2009 motion picture.
She previously explained: "It will be challenging for me. I can’t talk about it, but my part is a little different in each one. I’ll transform somewhat."
Due to COVID-19, 'Avatar 2' has been subject to many delays and has been pushed back until 2022.
Cameron recently admitted he is in awe of how the movie looks visually.
He said: "I deal with images in that world every single day and there are some days when I look at those images and say, 'This is really amazing'. I'm not patting myself on the back with the comment. It's a distillation of some of the best designers in the world and a great cast.
"But if you've got the script right and the design right and you have a great cast, then it's just an iterative process to get the whole thing shot and assembled."
There are four 'Avatar sequels' in the works.