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 married his second wife, Kathryn Harkin, a former Radio City Music Hall Rockette, on July 2, 1944. Marrying his second wife, ruined his relationship with his parents as his wife was a gentile and not a Jew.
Ivan Back, the press agent for the downtown night club Cafe Society, where Zero was hired as a comedian, nicknamed Sam Mostel Zero, as he said that he, Zero, was starting from nothing.
Zero married his first wife, Clara Sverd, a CCNY classmate, in 1939. The marriage did not last as Zero spent many hours in his studio with his friends and, he was unable to provide for her on the same level as her parents had. They separated in 1941 and got divorced in 1944.
Zero went to the City College of New York, a public college, where he belonged to the swimming team and the R.O.T.C. (Reserve Officers' Training Corps program.
Zero used to claim that he was nicknamed Zero because of his grades at school, however, in contract to his claim, he was in fact, an A student.
Zero spoke English, Yiddish, Italian and German.
Zero enjoyed painting and drawing from an early age and continued to do so throughout his life.
When Zero testified before the House Un-American Activities Committee on October 14, 1955 he told them that he was willing to discuss his own conduct but that he would be unable to name others as it is prohibited by his religion. His refusal to provide the committee with names resulted in him being blacklisted. Following his blacklisting he did not appear in movies for the next 11 years.
As per Zero's request, he did not have a funeral nor did he have a memorial service in his honor.
Zero drank so much coffee that at one point he was hospitalized for caffeine poisoning.
Mel Brooks was quoted saying that Zero could be wicked and cruel, and he could be almost sweet, loving, kind, generous, that he was uniquely gifted and more talented then any other actor he had worked with other than Sid Caesar.
The play Zero Hour, written and performed by Jim Brochu, was written about Zero Mostel's life.
Zero won a Tony Award for Rhinoceros in 1961 and for Fiddler On The Roof in 1964.
Zero was one of The 12 blacklisted by the house of Un-American activities commitee in the 1950s.
Mostel attended the City College of New York.
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?
Zero: (Commenting on Jim Henson's Muppets) He has the best possible actors. If you have a disagreement with them, you can always use them to wash your car.