My father forced me to stay up and watch Alien when I was nine. It terrified the living fuck out of me. The next week it was this, which was ten times worse. Of course I've seen them both about fifty times each now. Thanks dad.
Oh, he definitely is. There's no time after this scene and before the blood test scene (the part when it's revealed he's been assimilated) where he's apart from the main group, so he already is The Thing.