His films include "To Hell and Back", the biography of Audie Murphy, who was the most decorated American soldier of World War II; John Wayne's Vietnam war film "The Green Berets"; opposite Gregory Peck in the space story "Marooned", in which Janssen played an astronaut sent to rescue three stranded men in space, and "The Shoes of the Fisherman", as a television journalist in Rome reporting on the election of a new Pope (Anthony Quinn). He also played pilot Harry Walker in the 1973 action movie "Birds of Prey".