Brantly was traded to the Marlins in July 2012, and Castellanos is taking over this year as Detroit's regular third baseman.
"It's still great to see old friends, and teammates, but the bottom line is we've got a game today," said Brantly, who likely will open the season at Triple-A New Orleans.
Brantly is off to a solid Spring Training, going 7-for-14 entering Thursday. On Wednesday, he connected on his first Grapefruit League home run.
Brantly attributes his quick start to good preparation he developed while working in the offseason with Castellanos.
"I pretty much lived with my old teammate from Detroit this offseason," Brantly said. "Me and him have always worked pretty well together. We have similar hitting philosophies, even though it's from the right and left sides. We really were able to bear down this offseason and got some good work in."
Brantly is a left-handed hitter, while Castellanos bats from the right side.
Brantly also has a friendship with Tigers slugger Miguel Cabrera.
When Brantly was a prospect with Detroit, in Spring Training, he often was placed in a batting practice group with Cabrera.
"I had the opportunity to watch him a lot, and pick his brain when I could," Brantly said. "I did learn a lot from him, and a lot of the guys that I hit with over here. When I see him, I thank him, and wish him well and everything."