Jim clearly has very hairy arms, but in the closeup at the beginning of the rock and roll medley, the arms are clearly smoother.
Although there are several close-ups of a real guitarist playing to the soundtrack, James Best never looks the least convincing that he's actually fingering the notes heard in his two appearances as Jim.
The solo Jim plays in the jailhouse is very evidently on an electric guitar, not an acoustic.
When Jim Lindsey finally gets mad enough to play his guitar for Bobby Fleet, he is clearly seen playing only the rhythm portion of the music but is heard playing both rhythm *and* lead guitar at the same time and there is no other guitar player in the band, thus the audition. Additionally, the drummer in the cell who is seen only tapping his drum sticks on the cell bars is able to create the sound of a full drum kit.
In the coda, Andy and Barney walk Bobby Fleet to his car. In the background there is a three story building. A window on the 3rd floor is open showing nothing but sky beyond, revealing the building is a false front.
Jim Lindsey is playing an unplugged guitar, yet the electric guitar sounds as though it's plugged into an amplifier.