About the biggest change for Realmuto was having shortstop Miguel Rojas bring his first baseman's glove before the bottom of the third inning. Realmuto had singled and was stranded for the final out.
"It's different not having to go throw your gear on after the third out," the 26-year-old said. "It's kind of nice not having to do that."
Realmuto is one of the top catchers in the Majors, and the Marlins intend to keep it that way. But with Justin Bour on the disabled list with a right oblique strain, Realmuto is expected to get a few starts at first.
"I think it's a way to give him a day off and keep his bat in the lineup," Marlins manager Don Mattingly said. "But he's so athletic. That play he makes over his shoulder early, nobody makes on our club. And I think that's one of the advantages. You feel like he's as good as anybody we have over there. So, I think we'll see a little bit more of it to get him some at-bats."
Realmuto showed his athleticism in the second inning, running down Tyler Flowers' fly ball in short right field.
"I saw it go up," Realmuto said. "I put my head down and ran to the spot I thought it was going and peeked up, and it was right over my shoulder. It was something we worked on in Little League as an outfielder drill. You turn to the spot and run."
Before Sunday, Realmuto had played just two innings at first base -- on May 16 in a 12-2 loss to the Astros.
"I'm just trying to have fun with it," Realmuto said. "I haven't even taken a ground ball since Spring Training. I wasn't really sure how it was going to go, but I was just going to have fun with it, and it ended up working out."
On Sunday, Realmuto chipped in with a first-inning home run, which Statcast™ projected at 388 feet with an exit speed of 102.7 mph. For the season, his slash line is .306/.366/.489 with 13 home runs and 45 RBIs.
Because he is a threat at the plate, first base is another way to keep Realmuto in the lineup.
"He's still our catcher," Mattingly said. "He's not going to be a guy over there three days a week or anything like that. He's pretty much our catcher. But I think it is a way to get him off his legs, a day game like today."
Joe Frisaro has covered the Marlins for MLB.com since 2002. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.