Question: What American singer, film actor and comedian, suddenly stood up during a performance of Gershwin's Liza (All the Clouds'll Roll Away) and started singing?  
Sentence (from "Liza (All the Clouds'll Roll Away)"): It was introduced in 1929 by Ruby Keeler (as Dixie Dugan) in Florenz Ziegfeld's musical "Show Girl".
Relevant evidence: No