He told Entertainment Weekly: "I'm in the process of doing another pass through all three scripts right now ... Just refining. That's in parallel with the design process.
"The design process is very mature at this point. We've been designing for about a year and a half. All the characters, settings and creatures are all pretty much set."
Meanwhile, Sigourney Weaver previously teased her character would be making a return in the sequels, despite being fatally wounded in the first movie as she claimed "no one ever dies in science fiction".
She shared: "I just know I'm playing a really cool character so I'm kind of working on it to the extent of what I know. We haven't started (filming) yet, but when we do, we'll all probably disappear off the face of the earth for a year and a half."
