Zero often improvised while on stage. The audience enjoyed it but the situation often left the other actors, who were unprepared for his improvision, speechless.
Gene Wilder has mentioned that at the dinner party celebrating the release of the movie The Producers, Zeor Mostel switched Gene's place card with Dick Shawn's, hence, allowing Gene to sit at the main table.
Gene Wilder, who remained a close friend of Zero's until his death, mentioned in his autobiography, Kiss Me Like A Stranger that he was initially terrified of Zero. However, the first time they were introduced, Zero got up and gave him a big kiss. Gene said in his autobiography that he was very grateful for the lesson and, for giving him a ride to work everyday.
Zero: (on his first meeting with Jerome Robbins, after Jerome named him and others before the House Un-American Activities Committee) Hiya, Loose Lips!
Zero: (on Jerome Robbins) Jerome Robbins may be a talented choreographer, but he is no mensch. It's thanks to him and people like him that Jack Gilford and I and countless others were blacklisted during the McCarthy era. I never thought, during the 1950s, that someday I'd ever work with Jerome Robbins. I also never thought I'd work again, period.
Zero: (on his name) If people know your name, you're famous. If they only know your face, it doesn't count. It's the name. One reason I became Zero was so if I accomplished only a middle level of fame, I still wouldn't be a zero. If you know my name, how can I be a zero? Smart, huh?