Signed as an undrafted free agent in 2004, the catcher came up through the Cubs' organization and was named the starter this spring. Castillo debuted in 2010 and saw plenty of action last year, playing in 50 games.
Catchers have the widest range of responsibilities in baseball -- calling pitches, relaying signs to the defense, communicating with the umpire, covering the plate, throwing out runners, all while making sure balls don't get past them -- but Castillo doesn't let things get too stressful.
"This is a game, so what do you do in a game? You just go out and go have fun," Castillo said.
But just because it's fun doesn't mean it isn't work. Castillo arrives early every day, working to improve everything from blocking the ball to how he catches certain pitches, and that extra effort has been noticed.
"He's been working his tail off all spring, and it shows," said reliever Michael Bowden.
A first-round Draft pick by the Red Sox in 2005, Bowden got to know Castillo after coming to Chicago as a part of the Marlon Byrd trade last April.
"He's fun to throw to," Bowden added. "I think one of his best assets is attitude. He's very positive, he's fun to be around."
One of the major adaptations Castillo has made since being called up is working against Major League batters, which Castillo describes as "way different and way better" than those found in the Minors.
"You've got to make the adjustments back there," he said. "You've got to study them, like if you are in school. You've got to watch a lot of video. A lot of scouting reports, too. You've got to be on top of a lot of that to make your pitchers better."
Catchers also have to serve as an anchor for their battery partner, occasionally making trips to the mound to help the pitcher stay in the zone.
"Sometimes you just have to be there for them," Castillo said. "Just, 'Hey, how are you?' stuff like that, to give you a little time to breathe, and that's it."
It's during those times that Castillo's attitude can make a big impact.
"It's huge," Bowden said. "He comes out there and smiles at you and just says something positive. You know, that goes a long way with pitchers. It helps calm you down and just gives you confidence to go and get your next pitch."
Pitching is arguably the highest pressure position in baseball, but catching can't be far behind, in part because good catchers try to take some of that load off the pitcher's shoulders.
"If we give up a home run, they feel like they did something wrong, just like we did," Bowden said, "so they take a lot of responsibility. They take a lot of pride in calling the games, and like I said, he's done a very good job of doing that."
And pitchers appreciate battery partners who can put them at ease.
"Just having him behind the plate, you have a sense of comfort," Bowden said.