The concept of an "Alien" prequel helmed by director Ridley Scott dates back to 2002. It would take years to be a completed movie, due to 20th Century Fox's alternate ideas for the "Alien" movies, but eventually, Scott and Fox would resume talks over what would become "Prometheus."
Since "Prometheus" was going to use an entirely new cast, fresh actors would need to be hired, including Noomi Rapace as Dr. Shaw. Natalie Portman, Anne Hathaway, and Carey Mulligan were all frontrunners, but Rapace would eventually secure the role.
Screenwriter Damon Lindelof explained that the film needed to wrap up its own story and not just end where “Alien” began. He also noted that the ending's lack of concrete answers was intentional — a byproduct of how everyone involved wanted to leave audiences thinking.
Theron was initially approached to play the lead role, but losing out on that allowed her to portray the malevolent Meredith Vickers. This wasn't a part that was just handed over to Theron, as she managed to beat out other names for the part, most notably Angelina Jolie.
Many wondered whether the film was a reboot or a prequel. The film would make it clear in an epilogue that this was a prequel firmly set in the existing universe, but for the longest time, uncertainty hung over whether or not "Prometheus" was indeed a true "Alien" movie.