Originally, Ken Curtis used his Festus voice as a joke
The character's voice came out of a joke.
When viewers were introduced to Festus on Gunsmoke, neither they nor the show's creators had any idea of the lasting impact he'd have. At first, the Ken Curtis appearance was only intended to be a guest spot. However, he'd spend years on the show, and Curtis gladly admitted he'd have stayed on the show as long as he could after its cancellation.
Curtis spoke of his time on Gunsmoke to the Associated Press and said, "Oh my, it was such a great outfit." He also confessed, "I enjoyed every minute of every day. I wouldn't have cared if it had gone on for another 12 years. You don't find a home like that very often."
So, despite the character's longevity and Curtis's love for the series, he had no idea that he'd be playing the character for so long. According to an interview with the Fort Worth Star-Telegram, Curtis revealed that in his first audition for a Festus-type role, the character was quite different from the man we saw on our screens much later.
Curtis appeared in the film The Searchers, directed by John Ford, playing Charlie McCorry, a decidedly Festus-like character. However, Curtis confessed that in his original audition for the film, he took a different direction. He said, "I read it straight the first time."
The execution wasn't quite what Ford was looking for, but he knew that Curtis was capable of delivering the character he was looking for. Curtis said, "John Ford, who had heard me kidding around the set in my Festus talk, asked me to read it again." He said that Ford asked, "This time, use that dryland dialect."
When Curtis used his Festus voice, Ford was delighted and he won the role. Curtis, however, was less than pleased. He said, "It kind of shattered me at first."
Still, Curtis came to love the Fesus voice, no doubt thanks to all the success it brought him.