In his autobiography, Johnny Cash recalled meeting Elvis Presley in Memphis, circa 1954:
The first time I saw Elvis, singing from a flatbed truck at a Katz drugstore opening on Lamar Avenue, two or three hundred people, mostly teenage girls, had come out to see him. With just one single to his credit, he sang those two songs over and over. That's the first time I met him.
Although the two musicians were "never tight," they liked one another. Cash admired Presley's rhythm guitar playing and his showmanship. He writes: "Elvis was so good. Every show I did with him, I never missed the chance to stand in the wings and watch. We all did. He was that charismatic." Which brings us to the short, completely amusing clip found above.
According to the Pig River Records web site (a "comprehensive guide to music as it was 50 years ago"), this footage dates back to a 1959 tour. Cash was the opening act; Presley, the headliner. And each night, "Cash would impersonate his friend and touring partner, and then Elvis would come out and do the same. Two characters just having a good ol’ time whilst simultaneously creating the genre of rock and roll."
If you want to spend a little more time at the Cash-Presley nexus, I'd encourage you to listen to Million Dollar Quartet, a recording that captures Cash and Presley's impromptu jam session with Carl Perkins and Jerry Lee Lewis. It was recorded in 1956, at the Sun Record Studios in Memphis.