According to William Hanna and Joseph Barbera, Debbie Reynolds volunteered to perform the voice of Charlotte the spider for no salary, because of her love for the E.B. White book the movie is based on.
Originally, Tony Randall had signed on to voice Templeton in January 1972, and he recorded all Templeton's dialogue and songs. However, according to co-director Iwao Takamoto, when he and Joseph Barbera listened to Randall's vocal tracks, they realized that "Tony's delivery was a bit too sophisticated and his singing was too operatic", and that "he did not have enough of that raunchy feeling that the character required." Therefore, Barbera decided that Randall needed to be replaced, and so he invited Paul Lynde in to do a reading. Lynde was officially cast in March 1972.
"I Can Talk" is the only song of which no score is heard elsewhere in the film.
Hanna-Barbera pushed to have the film released in March so it wouldn't face competition from Disney's Robin Hood (1973), scheduled for release in November.
E.B. White deeply disliked how William Hanna and Joseph Barbera redesigned numerous scenes. He watched the finished version, then never watched it again, calling it a "travesty".