“I thought this whole trip was for her and I was just along for the ride,” Castleman said.
Instead, the teenager ended up singing the superstar's smash “All-American Girl” on stage as Underwood, Jackson and thousands of fans looked on and listened intently.
“I but she's not nervous at all,” quipped Underwood, who grinned broadly as Castleman began an impressive rendition of the upbeat anthem.
The fans clapped along, until the hopeful nailed the soaring chorus, then they burst into approving cheers.
“That was awesome,” Underwood said. “I gotta go. She can finish the show.”
Jackson added, “I think you've got what it takes to come and sing in front of me and my co-judges” and presented her with an “Idol” contestant number.
Castleman said she will return to Oklahoma City for the November “Idol” auditions.
In the meantime, she got to meet backstage the singer Jackson called “the greatest 'American Idol' to date.”
"Thank God even crazy dreams come true." Castleman posted on Twitter, quoting the lyrics of Underwood's song “Crazy Dreams.” “Thank you for this opportunity!”