It's mid-May, early in the season, and Johnson wants to tweak his pitching windup, to start with his hands much lower than they've been.
He stretches for a few minutes, then warms up his valuable left arm by playing long toss with a bullpen catcher about halfway down the left field line.
The catcher soon moves closer to Johnson, to about the regulation distance between a pitcher's mound and home plate 60 feet, six inches. He squats, places a white towel in front of him as a makeshift plate and dons a protective mask.
"If I come down a little lower here, maybe just an inch or so, I won't give up anything," Johnson tells Kniffin, meaning he doesn't want to tip off a hitter that he's gripped the ball for a fastball, slider or split-finger pitch.
The coach nods, letting the 38-year-old pitcher work things out for himself. Johnson continues to throw, but he's not satisfied with how it feels.
"You trying to think too much, R.J.?" Kniffin asks.
"I'm not trying to think," Johnson replies. "I'm just trying to throw it. I feel like I'm tilting too much with my fastballs. Just trying something different. How's it looking?"
"Good," Kniffin says.
In the dugout a few feet away, struggling young relief pitcher Jose Parra observes Johnson intently.
"Look at that guy," he says. "Going right from here to the Hall of Fame, and working like a dog."
Kniffin looks at his watch, lets Johnson throw a few more pitches, then informs him that his workout time his up. The coach stays behind after Johnson saunters back to the clubhouse.
"That's a professional pitcher right there," Kniffin says of the eight-time All-Star.
Like Johnson, Kniffin is left-handed; he's a former pro pitcher himself. Both have competitive mentalities that mesh well with the game they love.
But that's where obvious similarities end.
Johnson makes $13.3 million a year, more or less the remarkable going rate for elite pitchers.
Now 51, Kniffin never earned more than $3,000 a month as a player and then only in 1978, his last year in pro ball.
He labored in the minor leagues for 10 years far longer than most players but never got a chance at the big time. After it abruptly ended for him at age 27, a bitter Kniffin exiled himself from the sport for a decade.
Then, in 1988, he returned to the minors as a coach for a Wisconsin team. Thirteen years, seven more minor-league teams and a thousand bus rides later, he finally caught the professional break of a lifetime an offer to be pitching coach for the newly crowned World Champions.
The motley crew that Chuck Kniffin oversees is the centerpiece of what has become a most entertaining baseball team. Thousands of neophyte D-Backs fans have embraced the squad since last year's monumental World Series, tossing off names such as Kim, Spivey and Schilling with the same élan that Phoenix Suns fans once did for Johnson, Johnson and Johnson (Kevin, Frank and Eddie not Randy, who happens to be taller than all of them).
Notwithstanding the public-relations disaster that looms if the players strike later this summer, or the recent flap about the prevalence of steroids in the game, baseball in this winner-driven market is flourishing.
The dozen or so men on the D-Backs pitching staff (plus those on the disabled list) include hurlers from four nations with at least five different pitching styles. Their skill levels, experience and temperaments are equally diverse.
All are exceptionally well-paid: Kniffin's pitchers will collect about $52 million in salaries this year, $23 million of it going to Johnson and Curt Schilling.
Part of his unwritten job description is this: To know when and how to say something to one of his charges, and when to stay quiet and just let things happen.
"He's learning the mechanics of every pitcher here, and that's a whole lot of different mechanics," says Randy Johnson. "A good pitching coach is here to get you straightened out and back into that comfort zone, and really to head off things that are starting to happen before they happen."
That's the technical side. Kniffin is adamant that the mental part of the game is often more crucial to success at the major league level than are the mechanics.
"Nobody can help me with that, because I am who I am," Johnson says, before pausing briefly. "Then again, I've been around here for a long time. Other guys have different needs."