Elvis could act. Say what you like - no matter how inexperienced he was (no acting training at all), when given the chance, Elvis was a natural at acting. Watch King Creole, Wild in the Country, Follow That Dream, Flaming Star, Change Of Habit, and then try to say he couldn't act. You can't.
The plot/script of King Creole, while no Oscar winner, is better than average '50's fare and is certainly one of the best Elvis would ever get. King Creole is dark film noir set in the backdrop of the French Quarter in New Orleans, with Elvis as Danny Fisher, a twice high school dropout trying to find whatever means of a buck for himself, his sister and his unemployed father. Matthau is interesting as the very bad crime boss, and the unfortunate Ronnie is ably done by Carolyn Jones.
This film, with Elvis' three other pre-Army films, give us a wistful glimpse of what could have, and should have been, if he had been taken more seriously. If it hadn't been for his conniving manager, who exploited his career and took over much of his professional, creative, and personal life, Elvis may have had the respectable reputation as both competent actor and singer as Sinatra had. Sadly, this was never realized: his output as actor is remembered solely from the many horrible '60's musicals that Col. Parker forced him into.
If Elvis had truly pushed for better scripts like King Creole, he could easily have been the kind of actor more in line with the James Deans, Marlon Brandos, or Tony Curtises he admired. Instead, the rock n' roll rawness that was present in King Creole and Jailhouse Rock was gradually sterilized, until he became the polite, bumbling man reduced to singing "Old MacDonald Had a Farm" to children. It's not hard to see that his growing disappointment, so obvious in his later films, played it's part in his later demise.