The two met initially in 2011, when then-newcomer McCarthy headed to the Bay Area as a free agent and quickly made it his mission to befriend the seemingly quiet Cahill.
"I pestered him," McCarthy said. "Especially as starters, you're always around each other, on the same routine, facing the same lineups. I don't like when people just sit there, so I try to bring them out of their shell to find out all the weird stuff about them. That helps break the ice, and we just become friends."
While with the A's in 2012, McCarthy was in town for Cahill's Interleague start on "Star Wars Night" at Chase Field, which seemed very appropriate to the former Oakland starter.
"For Trevor's birthday this year, we got him a bunch of Star Wars stuff," said McCarthy. "When he was with the A's, we tried to get it out there just how into Star Wars he is and then how big of a deal it is that he was throwing on 'Star Wars Night' here. He's always trying to downplay it. He'll say, 'I'm trying to keep up my street cred.' I told him, 'Just tell everyone you love motorcycles and leather, with a Star Wars collection on the side.'"
While he is still a novice at social media, Cahill knows his friend is no stranger to connecting with fans -- admittedly sometimes at his expense.
"I know he's always talking about me on Twitter," said Cahill. "People come up to me all the time asking me about stuff, and I'm like, 'How do you know this?' Now I just assume it's him."
Laughing off his pal's antics, Cahill gives as good as he receives when it comes to the good-natured ribbing.
"Brandon's really into soccer," said Cahill. "It's something I gave him a hard time about a lot. He used to put it on the clubhouse TV, and everyone would yell at him. Soccer just isn't as popular here in the United States, and we definitely let him know that.
"Having him as a friend really helped us as teammates in Oakland, though. Getting input from a friend instead of just another player kind of gives you another perspective on how to work on things, like if something is going badly or needs to get switched up."
Despite being on separate teams in 2012, the two remained close. Cahill and his fiancée joined McCarthy and his wife for a vacation in the Bahamas last winter, and both players admit that maintaining a friendship on separate teams wasn't all that hard, just different.
"It really did hurt when he left," said McCarthy. "My wife and I got close to him and his fiancée. When he left, I just had this feeling like, 'Who am I going to talk to now?' It's an easier transition than you think it will be, but it was still tough, because we were getting really close with them. It was easy to maintain our friendship, though. We'd send each other texts all the time."
Now with McCarthy joining his friend in Sedona Red for 2013, the duo will get to see each other on a daily basis rather than just one-game reunions during the Cactus League in the spring.
"It could've been worse," said Cahill, after he was separated from his buddy following his trade to the D-backs last year. "I didn't get sent to a different country. It was just a different team and league."
Rachel Verbits is a member of the D-backs organization. This story was not subject to the approval of Major League Baseball or its clubs.