He shares a close friendship with tennis player Steffi Graf, who also makes an appearance (as herself) in the movie Otto – Der Außerfriesische. He has one child (“Benjamin Karl Otto Grigori Waalkes”) with his first wife Manuela Ebelt, and was later married to actress Eva Hassmann until November 2011, who co-starred with him in Otto – Der Katastrophenfilm.
He also works as a voice actor, providing, among others, the German voices of Mushu in Disney’s Mulan movies and Sid the Sloth in the Ice Age franchise.