Jerry Leiber and Mike Stoller wrote 4 songs ('Jailhouse Rock', 'I Want to Be Free', ' Treat Me Nice', and 'You're So Square') in 5 hours after their music publisher, under deadline, locked them in their hotel room.
Originally the choreographer, Alex Romero, created a dance for the song "Jailhouse Rock" that was in a style that was apropos for a more classically trained dancer than Elvis. When Mr. Romero realized that his plans for the number were never going to work, he asked Elvis how would he normally move to the song; thus, this is how Elvis became the uncredited choreographer for what could be considered his most famous dance number in all of his movies.
The film was not actually shot in widescreen. It was converted to Cinemascope in the final print after having been shot in a standard Academy ratio, much like some films which are "matted" after having been filmed in a normal size.
Elvis's band in the film is his real-life band including Scottie Moore on electric guitar and Bill Black on stand-up bass. Both have been with Elvis since his beginnings in Memphis at Sun Records. In the recording studio scenes, the piano player is Mike Stoller of the Lieber and Stoller songwriting team who wrote many of the major hit rock songs of the 50's.
In the lawn party scene, just after Vince embraces an unresponsive Peggy, the hired orchestra begins an instrumental number. The tune they're playing is "All I Do Is Dream Of You", from the 1934 film "Sadie McKee". This same tune, with vocal accompaniment, was featured in a series of television commercials for Bell Canada during the mid-1970s.