Question: Which film director also appeared on screen as an actor, Dziga Vertov or Frederic Zelnik?

full_context: Document 1: Frederic Zelnik: Frederic Zelnik (17 May 1885 - 29 November 1950) was one of the most important producers-directors of the German silent cinema. He also appeared on screen as an actor.
Document 2: A Sixth Part of the World: A Sixth Part of the World (Russian: Шестая часть мира , Shestaya Chast Mira ), sometimes referred to as The Sixth Part of the World, is a 1926 silent film directed by Dziga Vertov and produced by Kultkino (part of Sovkino). Through the travelogue format, it depicted the multitude of Soviet peoples in remote areas of USSR and detailed the entirety of the wealth of the Soviet land. Focusing on cultural and economic diversity, the film is in fact a call for unification in order to build a "complete socialist society". A mix between newsreel and found footage, Vertov edited sequences filmed by eight teams of kinoks ("kinoki") during their trips. According to Vertov, the film anticipates the coming of sound films by using a constant ""word-radio-theme"" in the intertitles. Thanks to "A Sixth Part of the World" and his following feature "The Eleventh Year" (1928), Vertov matures his style in which he will excel in his most famous film "Man with a Movie Camera" (1929).
Document 3: Zoltan Korda: Zoltan Korda (June 3, 1895 – October 13, 1961) was a Hungarian-born motion picture screenwriter, director and producer. He made his first film in Hungary in 1918, and worked with his brother Alexander Korda on film-making there and in London. They both moved to the United States in 1940 to Hollywood and the American film industry.
Document 4: Fred Paul: Fred Paul (1880 – 1967) was a Swiss-born British actor and film director. Paul was born in Lausanne in 1880 but moved to Britain at a young age. He was a prolific actor and director in the 1910s and 1920s, but his career dramatically declined with the arrival of sound films.
Document 5: Fred C. Newmeyer: Fred C. Newmeyer (August 9, 1888 – April 24, 1967) was an American actor, film director and film producer.
Document 6: Dziga Vertov: Dziga Vertov (Russian: Дзига Вертов ; born David Abelevich Kaufman, Russian: Дави́д А́белевич Ка́уфман , and also known as Denis Kaufman; 2 January 1896 – 12 February 1954) was a Soviet pioneer documentary film and newsreel director, as well as a cinema theorist. His filming practices and theories influenced the cinéma vérité style of documentary movie-making and the Dziga Vertov Group, a radical film-making cooperative which was active from 1968 to 1972.
Document 7: Kino-Eye: Kino-Eye (Anglophonic: Cine-Eye) is a film technique developed in Soviet Russia by Dziga Vertov. It was also the name of the movement and group that was defined by this technique. Kino-Eye was Vertov's means of capturing what he believed to be "inaccessible to the human eye"; that is, Kino-Eye films would not attempt to imitate how the human eye saw things. Rather, by assembling film fragments and editing them together in a form of montage, Kino-Eye hoped to activate a new type of perception by creating "a new filmic, i.e., media shaped, reality and a message or an illusion of a message - a semantic field." Distinct from narrative entertainment cinema forms or otherwise "acted" films, Kino-Eye sought to capture "life unawares" and edit it together in such a way that it would form a new, previously unseen truth.
Document 8: Dziga Vertov Group: The Dziga Vertov Group (French: "Groupe Dziga Vertov" ) was formed in 1968 by politically active filmmakers including Jean-Luc Godard and Jean-Pierre Gorin. Their films are defined primarily for Brechtian forms, Marxist ideology, and a lack of personal authorship. The group, named after 1920s-'30s Soviet filmmaker Dziga Vertov (1896-1954), was dissolved soon after the completion of 1972's "Letter to Jane".
Document 9: Robert Z. Leonard: Robert Zigler Leonard (October 7, 1889 – August 27, 1968) was an American film director, actor, producer, and screenwriter.
Document 10: Fred Zinn: Friedrich Wilhelm "Fred" Zinn was a volunteer American aviator of German descent who flew with French Armée de l'Air forces in World War I and an early pioneer of aerial photography for wartime reconnaissance and Military intelligence.
Document 11: Cinema of Ukraine: Ukraine has had an influence on the history of the cinema. Prominent Ukrainian directors include Oleksandr Dovzhenko, Dzyha Vertov and Serhiy Paradzhanov. Dovzhenko is often cited as one of the most important early Soviet filmmakers, as well as being a pioneer of Soviet montage theory and founding Dovzhenko Film Studios. In 1927 Dziga Vertov moved from Moscow to Ukraine. At the film studio VUFKU he made several avant-garde documentaries, among them «The Eleventh Year», «Man with a Movie Camera» and first Ukrainian documentary sound film «Enthusiasm (Symphony of the Donbass)». Paradzhanov was an Armenian film director and artist who made significant contributions to Ukrainian, Armenian and Georgian cinema; he invented his own cinematic style, Ukrainian poetic cinema, which was totally out of step with the guiding principles of socialist realism.
Document 12: Leopold Jessner: Leopold Jessner (3 March 1878–13 December 1945) was a noted producer and director of German Expressionist theater and cinema. His first film, "Hintertreppe" (1921), is considered a major turning point which paved the way for the later German Expressionist experiments of German filmmakers F.W. Murnau, Fritz Lang, and G.W. Pabst.
Document 13: Lev Kuleshov: Lev Vladimirovich Kuleshov (Russian: Лев Влади́мирович Кулешо́в ; 13 January [O.S. 1 January] 1899 – 29 March 1970) was a Russian and Soviet filmmaker and film theorist, one of the founders of the world's first film school, the Moscow Film School. He was given the title People's Artist of the RSFSR in 1969. He was intimately involved in development of the style of film making known as Soviet montage, especially it's psychological underpinning, including the use of editing and the cut to emotionally influence the audience, a principle known as the Kuleshov effect. He also developed the theory of creative geography, which is the use of the action around a cut to connect otherwise disparate settings into a cohesive narrative.
Document 14: Kinoks: The Kinoks ("kino-oki," meaning cinema-eyes) were a collective of Soviet filmmakers in 1920s Russia, based most notably around film editor Dziga Vertov (pseudonym Denis Kaufman). In 1919 Vertov and his future wife, the talented film editor Elizaveta Svilova, plus several other young filmmakers created a group called Kinoks ("kino-oki," meaning cinema-eyes). In 1922 they were joined by Mikhail Kaufman, who had just returned from the civil war. From 1922 to 1923 Vertov, Kaufman, and Svilova published a number of manifestos in avant-garde journals which clarified the Kinoks' positions vis-à-vis other leftist groups.
Document 15: Cinéma vérité: Cinéma vérité ( ; ] ; "truthful cinema") is a style of documentary filmmaking, invented by Jean Rouch, inspired by Dziga Vertov's theory about Kino-Pravda and influenced by Robert Flaherty’s films. It combines improvisation with the use of the camera to unveil truth or highlight subjects hidden behind crude reality.
ground_truth: Frederic Zelnik