Simplify the following text for a Grade 13 native English-speaking person.
Original: Famous movie critic Roger Ebert wrote that Buster Keaton 's `` extraordinary period from 1920 to 1929 , he worked without interruption on a series of films that make him , arguably , the greatest actor-director in the history of the movies . ''
Simplified: Critic Roger Ebert wrote of Keaton 's `` extraordinary period from 1920 to 1929 , ( when ) he worked without interruption on a series of films that make him , arguably , the greatest actor-director in the history of the movies . ''