If you looked closely in the film's credits, you would have noticed that Greenwood was credited for the role of "Cooper" in the film. As the Montreal Gazette reports, "Cooper" was the nickname the production gave the alien monster on-set, and Greenwood -- who worked with Abrams on the Star Trek film -- completed performance capture work for the extraterrestrial creature so that his facial expressions could appear lifelike, especially in that final scene between the monster and Joe, played by Joel Courtney.
Said the actor about his Super 8 experience:
"It's not your typical acting exercise at all, because you're really the only person in this room with all these cameras in it. It was a very weird thing to offer up all this big emotion while the rest of the people in the room are picking away at the craft services table. And you never really how it will turn out because it's all left in the hands of the artists and animators who are putting your work together."
Fun. For what it's worth, "Cooper" did look kinda like Bruce Greenwood. Sorta. Feel free to discuss more Super 8 spoilers at length right here.
ยท Bruce Greenwood is scariest character you won't see in 'Super 8' [Montreal Gazette via CinemaBlend]