Robin Williams (18 images)

Robert De Niro (L), Billy Crystal, and Robin Williams stand on stage before Crystal receives his Mark Twain Prize for American Humor at the Kennedy Center in Washington on October 11, 2007. (UPI Photo/Scott Suchman/Kennedy Center)..

Actor Robin Williams leaves the Ed Sullivan theater where he was a guest on David Letterman's "Late Show" on January 2, 2008 in New York. "Late Night" is resuming production after Letterman made an agreement with the writers' guild who have been on strike for almost two months. (UPI Photo/Monika Graff)

Robin Williams (C) poses for photos with service members assigned to Kuwait during the USO Holiday Tour on December 18, 2007. Along with Williams, tour host Chairman of the Joint Chiefs of Staff, Navy Adm. Mike Mullen was joined by comedian Lewis Black, 7-time Tour de France champion Lance Armstrong, award winning recording artist Kid Rock and Miss USA Rachel Smith on the 15-stop, 7-country tour thanking the forward deployed troops for their sacrifice and service. (UPI Photo/Chad J. McNeeley/Department of Defense)

Robin Williams arrives for the 10th annual Mark Twain Prize for American Humor, which will be awarded to comedian Billy Crystal, at the Kennedy Center in Washington on October 11, 2007. (UPI Photo/Alexis C. Glenn)..