A really, really good actor. It's criminal he's not known better. From ALIEN alone, he was one of the main elements that pushed everyone to do better---to exhausting and frustrating levels. Coming from that time of naturalism and method, he'd riff with Stanton and the others before, during, and after the cameras rolled just to get everyone into that headspace for the characters. It's even more incredible to think that there was hardly any background or descriptions provided in the original script for any of the Nostromo crew, as well.