It is often said the crew was able to get Mister Ed to move his mouth by applying peanut butter to his gums in order for him to try to remove it by moving his lips. However, Alan Young said in 2004 that he had started the story himself.
In another interview, Young said, "Al Simon and Arthur Lubin, the producers, suggested we keep the method a secret because they thought kids would be disappointed if they found out the technical details of how it was done, so I made up the peanut butter story, and everyone bought it.
It was initially done by putting a piece of nylon thread in his mouth. But Ed actually learned to move his lips on cue when the trainer touched his hoof. In fact, he soon learned to do it when I stopped talking during a scene! Ed was very smart."
Others argued that examination of Mister Ed footage shows Ed's handler pulling strings to make him talk, and that this method was at work at least some of the time. Young later said during an interview for the Archive of American Television (AAT) that a nylon string was tied to the halter and the loose end inserted under his lip to make Ed talk, saying that he had used the peanut butter fable for years in radio interviews instead of telling the truth.
The loose thread can be seen tied to the halter, and it is clearly not taut as it would be if it were being pulled. Young also states in the AAT interview that after the first season, Ed did not need the nylon – Alan and trainer Les were out riding one day and Les started laughing, telling Alan to look at Ed, who was moving his lips every time they stopped talking, as if attempting to join in the conversation.
This difference is visible when comparing first season episodes to later ones, as it is clear that early on he is working the irritating string out, sometimes working his tongue in the attempt too, and later on he tends to only move his upper lip, and appears to watch Alan Young closely, waiting for him to finish his lines before twitching his lip.
Young added in the Archive interview that Ed saw the trainer as the disciplinarian, or father figure, and when scolded for missing a cue, would go to Alan for comfort, like a mother figure, which Les said was a good thing