Zira's first line in the presence of humans, "Because I loathe bananas!", is a reference to Kim Hunter's real distaste for this fruit, which originated during the filming of the first Planet of the Apes (1968). The actors portraying apes were required to keep their makeup on during breaks in order to save time, so the rest of the crew often called them monkeys and offered bananas to mock them.
All five original "Planet of the Apes" movies were #1 at the U.S. box office when released. "Escape from the Planet of the Apes" spent one week as the #1 top grossing film: the week of May 23, 1971 it made $4,294,942.
The trivia items below may give away important plot points.
Sal Mineo agreed to appear in the film because he hoped it would restart his career, much as the first "Apes" film had done for Roddy McDowall. However, when Mineo found the make-up uncomfortable, the script was re-written to kill his character off earlier than planned. This was Mineo's final theatrical film before he was murdered on February 12, 1976 at the age of 37.
For the death scene, Cornelius and Zira were originally going to be ripped apart by a pack of Doberman Pinchers (the feared dog from that decade) by Doctor Hasslein, but producers thought the scene would be far too gruesome, so they were killed by gunshots instead.
This film contradicts the prior films in a small but significant way. In the previous films, it was established that the ape society (other than "keeper of the faith" Dr. Zaius) had completely forgotten that an intelligent human society had preceded the ape civilization. But in this film, Cornelius says his people knew that dumb apes had once served human masters, until the apes revolted under a defiant leader named Aldo. Interestingly, the following sequels carefully edit Cornelius' comments to omit the name of Aldo, and leave the implication that Caesar is the revolutionary leader. The fifth film, Battle for the Planet of the Apes (1973), includes a central character named Aldo, but that film is set after the ape revolution, and clearly that Aldo is not the one Cornelius referred to.