Treat Williams was just 19 when he played the head hippie, Berger, in Hair, the 1979 movie musical based on the hit Broadway show of 1968. Williams is now 67, and while the sun has long since set on the Age of Aquarius, the film continues to resonate: long locks are practically de rigueur and the songs are cultural touchstones. Williams and his co-star, Annie Golden, sit for a screening of Hair, followed by a Q and A with audience members. —C.J.F.