Was Paul Giamatti in this?

The only "dude with a beard in one scene" I've come across is the doctor in the hospital scene who walks in on them all as Adam's sister is trying to relocate Mischa's arm. The guy has a receding hairline, beard, and glasses. I don't see the similarity, but I can see how he has those characteristics in common with the Paul Giamatti of today.

If this is the person to whom you are referring, I can say pretty confidently that it is NOT Paul Giamatti. Paul Giamatti would have been 20 when this film was released, and probably 19 or so when it was filmed. That doctor is at least mid-40s in 1987. Any resemblance (if it exists) would be to the current day Paul Giamatti, at nearly 50. The actor who played the doctor doesn't appear to be credited anywhere.

There could be another "dude with a beard" who appears in one scene that I might be missing. :)