Now, you've got to keep in mind that Elvis Presley was probably, innately, the most introverted person that came into that studio. Because he didn't play with bands. He didn't go to this little club and pick and grin. All he did was sit with his guitar on the side of his bed at home.
From what I have read Elvis had a little more experience than just sitting on his bed. He played at school, he played on the porch in the projects-but Elvis was not just introverted because of his lack of professional experience IMO. Phillips also mentioned a couple times he saw in Elvis the same traits of the black people of the 1950s south- almost an inferiority complex. Dirt Poor, wrong side of the tracks type complex.