Sir Ridley Scott admitted he was not particularly "a science-fiction fanatic" as he arrived in London for the premiere of Prometheus - a prequel to his groundbreaking 1979 film Alien.
Stars including Charlize Theron, Guy Pearce and Michael Fassbender joined the director at the event in Leicester Square.
Sir Ridley said he had been looking to return to the genre for a while before he hit on the right idea.
He said: "I'd been looking for something science fiction to do and couldn't really settle on anything and this thing was niggling me from the very first film, here was this question that was never answered.
"I've never really been a science fiction fanatic. When it's good, a good film is a good film whether it's big, small or whatever."
The Girl With The Dragon Tattoo actress Noomi Rapace stars in Prometheus as an astrophysicist who uncovers a clue confirming the existence of an extra-terrestrial race called the Engineers and joins a voyage to their home planet to find out more.
Charlize said making the film had been "really intimate" and said she enjoyed playing the slightly cold executive Meredith Vickers.
She said: "I think I'm somewhat attracted to people who don't make it so easy. They're not just handing their souls over - they make you work a little bit to get to know them, to get to understand them, to even like them and that to me feels real."
Her co-star, Logan Marshall-Green, said he had been a fan of the original Alien film, starring Sigourney Weaver, since his mother showed him it as a lesson in feminism.
He said: "My mum actually bought us a VHS when I was young. She wanted my brother and me to see Sigourney as a hero, a woman as a hero."