Bolaji Badejo was a non-actor discovered by casting agents in a bar prior to the start of Alien filming. Standing 7'2', the Nigerian man was determined (rightfully as this video proves) by Scott to be proportioned in a way that could easily be turned inhuman.
The fear had been no matter what, the Alien would just look like a man in a rubber suit, but even this early test footage shows how well Badejo was at subtle menace.
To prep for the role, Bolaji took mime classes and tai chi to acquire the flowing movement of the Alien species.