From the Autumn 1979 issue of Cinefantastique magazine:
The Alien you don’t get to see in ALIEN was played by 6’10”, 26-year-old Nigerian Bolaji Badejo. Bolaji is a student of graphic arts in London, and has travelled extensively with his parents: to Ethiopia where he studied fine arts; and to the United States, including a three year stay in San Francisco. He landed the role of “The Alien” purely by accident, a turn of events that reads like a publicity agent’s tall tale. The production had apparently put out a casting call for a very tall, very thin actor. Bolaji bumped into agent Peter Archer while having a drink in a London West End pub. Archer thought of ALIEN as soon as he spotted Bolaji, and offered him the chance to try out for the part.
“As soon as I walked in,” said Bolaji, “Ridley Scott knew he’d found the right person.” Scott had been looking at basketball players, and had tested Peter Mayhew [Star Wars‘ Chewbacca] for the Alien, but it was Badejo’s combination of height, slimness and an erect posture that cinched him the part. Bolaji was signed for the part in May, manufacture of the suit began, and the filming of the Alien scenes started in August at Shepperton.
Ridley Scott originally intended Bolaji to be part of a team of three artists needed to play the Alien, including a mime specialist and a karate expert. When other experts of Bolaji’s unique proportions could not be found, a stuntman was substituted for the dangerous and physically grueling action and Bolaji began to take miming lessons. Most of the footage shot of the Alien didn’t work, but there is one brief cut of Bolaji going through one of his miming routines in the suit, in the sequence where he attacks Veronica Cartwright. ”The idea,” says Bolaji, “was that the creature was supposed to be graceful as well as vicious, requiring slow, deliberate movements. But there was some action I had to do pretty quick. I remember having to kick Yaphet Kotto, throw him against the wall, and rush up to him. Veronica Cartwright was really terrified. After I fling Yaphet Kotto back with my tail, I turn to go after her, there’s blood in my mouth, and she was incredible. It wasn’t acting. She was scared.” (via)