They call it casting against type, having an actor play a part that he just doesn't seem to fit. Christopher Knight, who played Peter Brady on 117 episodes of "The Brady Bunch" without ever suffering a pimple, is now an anti-acne advocate for the American Academy of Dermatology. "The difference between acne and a lot of other problems we go through is that we can do something about this," said Knight, 43, to ABC News.com. "But many people just suffer." The point of his "Healthy Skin, Healthy Outlook" campaign is that acne can demolish self-esteem and contribute to depression, and over 40% of teenagers suffer from skin problems bad enough to require medical treatment. "I think I can get an important message across," he says. "People of all ages are always coming up to me and saying, 'I grew up with you.' I have an opportunity to speak to people about something very real." Knight left show business to sell computer equipment, but "The Brady Bunch" never left him. The show has been syndicated in 55 countries and been recycled into two movies, three television specials and countless fan conventions. As an adult, he was diagnosed with attention deficit disorder after spending much of his childhood struggling to learn his lines. "You can compare ADD to a skin disorder in that it can ruin a kid's self-image if he or she doesn't deal with it," he said, still grateful for the show that continues to get him recognized on the street. "It was the role of a lifetime for me."