The best performances in the movie are, by far, Ronny Cox [Apples' Way, Robocop, Total Recall] and Bibi Besch [Tremors, Star Trek II: The Wrath of Khan] who play the boy's parents. Both have always been great character actors and they really shine in this movie. Unfortunately Paul Clemens, who plays their son, is not nearly as good an actor as Cox or Besch. This definitely strips the film of some of its power. The townspeople are creepy enough and remind me of a Stephen King movie where everyone is just a tad bit off. They are fine victims when the boy begins his reign of terror and flesh munching.
I can't get too excited about The Beast Within. It's not a bad film but with a little more work it could have been a much better film.
For more info check out the film's entry in IMDB.