According to theater lore, when Valentin suggested to Brecht how to depict soldiers in battle, he planted the seed for Brecht's notions of Epic Theater. Walter Benjamin relates the story:
"Brecht in turn quoted the moment at which the idea of Epic Theater first came into his head. It happened at a rehearsal for the Munich production of Edward II (1924). The battle in the play is supposed to occupy the stage for three-quarters of an hour. Brecht couldn't stage-manage the soldiers, and neither could his production assistant. Finally he turned in despair to Karl Valentin, at that time one of his closest friends, who was attending the rehearsal, and asked him: 'Well, what is it? What's the truth about these soldiers? What about them?' Valentin: 'They're pale, they're scared, that's what!' The remark settled the issue, Brecht adding: 'They're tired.' Whereupon the soldiers' faces were thickly made up with chalk, and that was the day the production's style was determined."The unrealistic, chalky whiteness of the soldiers' faces is often cited as one of Brecht's first uses of an "alienation effect."
Valentin also starred in a short silent film written by Brecht: Mysteries of a Barbershop (1923). [I haven't been able to turn up a copy of this film, but I'll keep at it...]
There are a few clips of Valentin and Karlstadt available on YouTube, but they are rather poor quality (and aren't translated), and I don't think they capture the more ironic tone of their 1920s cabaret acts. These photographs, however, may give some idea of Valentin's stage personae during those years.