Mickey Shaughnessy
I saw "Jailhouse Rock" for the first time only recently. I thought it was great fun, with a certain element of seriousness about it as well. It was a brilliant vehicle for Presley, who was a big star but a novice actor (something he would remain for the rest of his film career). So they kept the general story something similar to his (minus the manslaughter & prison), something familiar in a certain way.
But I was particularly struck by the performance of Mickey Shaughnessy, who was simply brilliant. He kept Elvis' character afloat when it was in danger of sinking under the weight of inadequate acting abilities.