Sean Penn, star of the new movie "I Am Sam" (in which he plays a mentally disabled father who battles to retain custody of his daughter), screened his movie and spoke to a group of 450 Washington, D.C., high school students on Tuesday, reports PEOPLE. About one-third of his audience was mentally challenged, and the get-together was sponsored by the Special Olympics. Penn, 41, said referring to Joseph Rosenberg and Brad Silverman, his mentally challenged costars in "Sam" (who were in attendance): "I hadn't been very exposed to the world of these folks here until I worked on the movie." Penn also said that meeting mentally handicapped individuals helps to break down barriers of discrimination and fear. "It becomes so much about being exposed to the point of being comfortable. I think once people feel comfortable, they're open to learning and accepting much more." Silverman, who at one point broke down in tears as he thanked the members of the audience for their interest, said that in preparing for the film, Penn came to visit L.A. Goal, a California-based non-profit that serves adults with disabilities. There, the actor helped fold T-shirts and performed other duties. "I Am Sam," which opens Jan. 25, also stars Michelle Pfeiffer as the attorney who defends Sam (Penn) in his custody battle. Speaking about her role in the film on Wednesday morning's edition of "Good Morning America," the star, 42, told Diane Sawyer of her own subconscious biases against the mentally disabled. "I was really faced with prejudices that I didn't really know that I had, and I think like most people I just never really stopped to think about it," said Pfeiffer. "I realized that as a child, the only thing that's really ever said about mentally challenged people is 'Don't stare.' And so they go through life either being gawked at, or being invisible."