[Ruy Dias de Bivar]

Notice that Mr Ed and Francis the talking mule never talk unless the halter is on. The trainer is giving them physical cues to move their lips through the halters.

[Opinionated Blowhard]

Notice that Mr Ed and Francis the talking mule never talk unless the halter is on.

I always thought that they put something like a nylon thread in the horse's or mule's mouth to irritate it and make it twitch and give the appearance of talking.