"In front of everyone, he said, 'What do you think about pitching?' and I said, 'Yes,'" Castillo said.
He still likes to play shortstop but Castillo switched to pitching in 2010. His first game on the mound was an intrasquad game.
"I was kind of nervous," Castillo said. "It was my first time. I did three, four sides in the bullpen before a game. Now, I enjoy pitching."
In 2010, he was a combined 3-2 in 17 games with a 2.35 ERA at three levels in the Minors, and last season, he was 4-2 with a 2.54 ERA in 21 games at Class A Lakewood.
In December, the Cubs selected Castillo in the Rule 5 Draft. He's now a candidate for their bullpen, and the 22-year-old has impressed manager Dale Sveum.
"His makeup was OK, his stuff was obviously good enough to give it a shot and give it a look," Sveum said. "He's doing well, but I think his preparation and his makeup and the way he's handled himself on the mound is a little more important for a Rule 5 kid than just their stuff alone.
"He's been able to throw strikes," Sveum said. "His poise on the mound and the way he handles PFPs [pitchers' fielding practice] and athleticism and competitiveness has been far and above a lot of other Rule 5 guys I've been around over the years."
The Cubs paid the Phillies $50,000 for taking Castillo in the Rule 5 Draft and must keep him on the 25-man roster this season. He has to be active for a minimum of 90 days. If he does not remain active, Castillo will be offered back to the Phillies for $25,000.
Castillo understands the Rule 5 rules.
"Everywhere I go, I try to do my best," he said. "That's what I"m doing now. I hope to make the team. That's my goal."
It would be a huge leap from Class A Lakewood to the big leagues.
"It's kind of different, but it's the same -- it's baseball," Castillo said.