Peter Weller
I saw this movie around the same time I saw Buckaroo Banzai in 1984, and Peter Weller really impressed me--to play two such different characters, one goofy (Banzai) and one extremely creepy (Firstborn).
He turned up later in RoboCop and its sequel as well as in B-movies like Shakedown and Leviathan. I always thought he was underused. Maybe he was picky. He was perfect as Murphy and RoboCop even though you only get his voice as Robo.
For any Weller fans out there, try seeking out Shoot the Moon with Diane Keaton. His character in that one is sort of a passive/aggressive version of his character in Firstborn (until the finale). Too bad it's not on DVD.
Well I gotta go now Duane.....I'm due back on the planet Earthshare