Between takes, Zero Mostel clowned around throughout the production. He dressed himself in a red tunic and danced the Flamenco, fondled statues of cherubs, and batted his eyes for at the women in the cast.
It was always considered essential to the film that Zero Mostel should repeat his Broadway success in the leading role, but the actor demanded director approval before he would agree to take part. He submitted a shortlist of five names, any of whom he would accept as director - Orson Welles, Charles Chaplin, Jean Renoir, Richard Lester and Seth Holt. Dick Lester got the job.
Since movie musicals were losing popularity in the late 1960s, most of the play's songs were cut, including Zero Mostel's popular tour de force "Free", which would've been sung immediately after he fell out of the tree upon making his deal with Hero.
Jack Gilford was nominated for the 1963 Tony Award (New York City) for Supporting or Features Actor in a Musical for "A Funny Happened On The Way To The Forum" as Hysterium which he recreated in the movie version.
The ever-present flies in the end sequence are an in-reference to the actual flies that plagued the production. Fruit and vegetables were present throughout the sets on location in Spain, and were often left to rot in the sun at the end of the shooting day.
During filming, actor Phil Silvers locked himself in his trailer and refused to come out. Director Richard Lester asked Jon Pertwee to step into the role of Lycus, as he already knew the part after performing it on stage. Once Silvers heard he had been replaced, he came out of his trailer, bursting with new enthusiasm for the project. Pertwee was given the (much) smaller role of Crassus as compensation.
Zero Mostel and Jack Gilford reprised their roles from the original Broadway production. Phil Silvers had been offered the role of Pseudolus on Broadway, but turned it down because he couldn't wear his glasses, and feared an onstage accident. Silvers went on to play Pseudolus in a 1972 revival.
Every actor who has opened in the role of Pseudolus on Broadway (Zero Mostel, Phil Silvers and Nathan Lane) won a Best Actor Tony for their performance. In addition, Jason Alexander, who performed as Pseudolus in one scene in "Jerome Robbins' Broadway", also won a Tony for Best Actor in a Musical.
The original Broadway production of "A Funny Thing Happened on the Way to the Forum" opened at the Alvin Theater on May 1, 1962, ran for 964 performances and won the 1963 Tony Awards for the Best Musical and Best Book.