Glen was the hottest performer in the entertainment world when they cast him for True Grit. His recording career was in the stratosphere with those early hits, and his variety show on CBS was pulling strong ratings as well. Only logical that the producers would want to capitalize on his growing fame—and Campbell’s handlers would try to get him in a John Wayne movie.
Of course, Glen had few rivals as a picker. He made a lot of money as a member of the legendary “Wrecking Crew,” the studio musicians who played on (seemingly) every hit record made in Los Angeles in the early-to-mid 1960s. Campbell played on as many as 300 records in 1963 alone. Not bad for a kid from Delight, Arkansas who never learned to read music. But all a producer or arranger had to do was tell Glen what they were looking for (or run through the song once), and he delivered.
I'll bet he knew the Nashville Number System.