Castillo, 24, and Steve Clevenger, 26, are the finalists for the backup catcher position. It's not a glamorous job. Whoever gets it will play about once a week and most day games after night games to give Geovany Soto a breather.
Both Castillo and Clevenger are qualified and both have hit well this spring. Castillo was batting .324, while Clevenger was hitting .314 and each had hit two home runs.
"These guys could play every day for a lot of teams," Cubs manager Dale Sveum said. "These guys are well above average catchers."
Neither was a catcher when they began playing baseball. Clevenger was a converted shortstop, while Castillo was a third baseman. Both still have infielder gloves in their lockers just in case.
Castillo has slightly more big league experience, getting called up in 2010 and again last season. But a hamstring injury slowed him and he spent the offseason strengthening his legs. Last year, he pulled his right hamstring three times. Instead of playing in the Dominican Winter League, Castillo did his exercises.
"This is more important than winter ball," he said of the big leagues. "I've got to keep my body healthy."
A Phillies scout initially spotted Castillo and suggested he look into switching to catcher.
"He said, 'You've got really good hands, a really good arm and you can hit, but you can't run and you're really slow and you're not tall,'" Castillo said. "He said, 'Can you throw to second?' I said, 'sure but I don't know how to do it.'"
So, Castillo squatted behind home plate, then threw five balls from home to second base. The scout told Castillo his time was above average.
"But I don't like to catch," Castillo said. "I stopped playing baseball. My manager wanted me to catch and I didn't want to."
He was 16 years old then. Not playing baseball in the Dominican is almost unheard of, but Castillo took time off and went to school.
"I was really frustrated," he said. "This was the only job I had and I didn't want to catch. My mom and my sister and my father, they talked to me, and said, 'Let's play, let's go.'"
If he played, if he made it to the big leagues, Castillo could possibly earn enough money to take care of his family back home in San Isidro.
"I started to play shortstop again, but it was hard for me," Castillo said. "All the scouts saw me and they wanted me to be a catcher and I didn't want to do it."
Jose Martinez, a buscon, or scout, in the Dominican, started coming over to Castillo's home to practice with the teenager.
"He tried to make me a catcher, but at the same time, he hit ground balls," Castillo said. "I had two tryouts. I didn't know how to block, I didn't know how to catch. In both tryouts, they wanted to sign me. I said, 'That's my dream, I want to be a professional baseball player.' I figured, well, I'll do it."
He signed with the Cubs in 2004 and has come to realize catching is the best position for him.
"It's a tough position," he said. "Now, I love it. I know what's going on. You have to make all the decisions most of the time. You have the game in front of you, you have to be in the game. If you said to me, 'Do you want to play infield?' I'd say, 'No, I'm a catcher.'"
Cubs pitching coach Chris Bosio and bench coach Jamie Quirk have talked to Castillo nonstop about pitch calling.
"They're always asking me, 'What do you think in that situation?'" Castillo said. "I have an idea of the gameplan, but you can never stop learning. You're going to see new hitters, other hitters. I'm hungry to do this. I want to move up."
On Tuesday, he was tested against the Reds. The Cubs snapped a 3-3 tie in the ninth on Castillo's leadoff homer and eventually won, 7-4. But Castillo had to handle a long ninth behind the plate.
"Welly had one heck of a game [Tuesday] and had to deal with that last inning, blocking a lot of sliders," Sveum said. "Getting through that last inning wasn't easy and he handled it well and did a great job."
Castillo remembers the first game he caught.
"I got hit a lot of times on my neck," he said, chuckling. "I don't know how many times my thumb popped out [of the mitt]. It was not a good experience. I laugh now, because I know where I was and I know where I am now. You never forget where you come from, no matter how high you are or who you are. That's the only thing that my mom and my dad told me: Never forget where you came from. No matter how much money you have, don't forget where you come from."
And if he gets the call to the big leagues, Castillo will place a call to Martinez.
"I will. I appreciate what he did," Castillo said.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.