The big man from South Florida then turned serious, a personality trait more common with Mathis. They've shared the job, and a lot of good times, the past 2 1/2 seasons in Anaheim.
"He's trying to take my job, and I'm trying to take his," Napoli said. "But we're doing it in a good way. We never pull against each other. Hey, we do everything together. How could we do that?"
Napoli was born to hit. He's disciplined and powerful, a force of nature when he's locked in and feeling good about himself. Playing 140 games, he'd likely flirt with 30 homers and 100 RBIs while inflating his on-base percentage with free passes distributed by concerned pitchers.
But Napoli's catching skills -- and his throwing, most visibly -- need constant vigilance. Mathis is always there with a helping hand.
This is where Mathis flourishes, with the tools of intelligence. When he is throwing accurate darts, as he has this spring, he's about as good as it gets defensively.
It is his bat that has kept Mathis from taking his place as a full-service force. That is where Napoli comes in with some technical advice on occasion but mostly with words, trying to infuse his buddy with his own self-assurance.
Mathis has been working with hitting coach Mickey Hatcher on incorporating a loading mechanism into his swing, designed to slow him down.
Intensely competitive -- Florida State University offered him a full football ride out of Marianna (Fla.) High School -- Mathis falls into traps, and pitchers' counts, when he is lunging at pitches out of the strike zone.
"I got to the point where I was so jumpy, I was not letting the ball travel," Mathis said. "Mickey said, `I want you to try something.' It was a rocking motion. We've talked about it a lot, but I didn't apply it until late in the  season.
"Loading, I feel myself doing a better job of holding myself back. I'm hitting the ball better the other way. That's something I'm constantly working on."
Mathis unloaded a Napoli-like shot on Monday at Camelback Ranch against the Dodgers, his first homer of the spring. Napoli leads the club with four homers in 29 at-bats.
In his first at-bat on Tuesday against Mariners' Ryan Rowland-Smith, Mathis was robbed by Ichiro Suzuki nearing the fence in right with an implausible catch reminiscent of Willie Mays' unforgettable play on Vic Wertz in the 1954 World Series. Mathis had another extra-base hit stolen by third baseman Jose Lopez.
After hitting .211 in the '09 regular season, Mathis erupted in postseason play, hitting .533 with eight hits (five doubles) in 15 at-bats.
Napoli visited Mathis in his winter home, a state-of-the-art barn complete with a batting cage, and reinforced all the good feelings Mathis took from October.
"I don't think it's anything to do with his swing," Napoli said in reference to Mathis' past offensive struggles. "Jeff's got a pretty swing.
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"We talk about hitting all the time. It's more about keeping his head down through his swing. I keep telling him, `Keep your head down.' When he's going after certain pitches, his swing takes his head away.
"What he needs to concentrate on is seeing contact off the bat. When you're hitting off a tee, you're focused on seeing the ball as you make contact. When I start struggling, I'm not seeing the ball. When I'm comfortable and confident, I don't worry about anything. I just hit."
This basic philosophy applies to Napoli and his throwing issues.
"I have to separate the technical stuff and just play the game," he said. "I've struggled with that in my career. If I'm taking infield and do something right, I can say, `OK, there it is. Take that in the game and have some fun.'
"Instead of thinking, `If I don't do good catching, I'm not going to play,' I have to just go out and have fun."
Mathis, who has a quick release and plus-plus arm strength according to manager Mike Scioscia, frequently offers tips to Napoli.
"We talk a bunch about that," Mathis said. "He asks me questions, and I try to give him my input. We're always bouncing things off each other that can help. It's like, `Here's something, whatever you can take out of this ... '
"I think Nap and I have a lot of respect for each other and we want the other guy to succeed. We both want to play, but we know only one of us can catch at a time."
Scioscia maintains he has four Major League-caliber catchers, putting Bobby Wilson and Ryan Budde in that category. But Napoli and Mathis leave little time for anyone else.
"It's the same as the last couple years," Scioscia said. "It's worked out that it's been a split. I know those guys have the ability to go out and catch 130 games each. Why they haven't done it is because there have been some rough spots on the offensive or defensive end."
He didn't have to spell out which receiver had which flaws.
What's most remarkable about Mathis and Napoli -- more so than Mathis' defense and Napoli's power -- is their ability to blend friendship with competition in the spirit of cooperation.