On the mound, he's always had a pretty obvious load of it. He overwhelmed hitters at the high school level, going 10-2 with 153 strikeouts in 77 2/3 innings pitched during his senior season at Brownwood High School in Texas. Among the impressed were the Cardinals, who then snagged Miller with the 19th overall pick in the 2009 First-Year Player Draft.
That mound maturity, perhaps better defined as presence, hasn't escaped Miller in the Minors either. He had a solid 2010 campaign in low-A and got even better during moves to high-A and Double-A in 2011. The Cardinals tapped him as the organization's Minor League Pitcher of the Year in both seasons.
Off the mound, though, maturity hasn't always been so natural. With no choice but to grow up in the spotlight after becoming a teenage millionaire, Miller endured his biggest hiccup when he was suspended by the organization last summer for his reported involvement in a pair of alcohol-related incidents.
The suspension ended up not being all that lengthy. The impact, however, most certainly was.
"Being away from my team and being away from the field and not able to play really, really was an eye-opener," the 21-year-old Miller said on Monday, his second day at Major League camp. "I became better through the adversity. It's something that I've learned a lesson from, and I thank the Cardinals for what they did."
The Cardinals are hopeful that Miller, who slimmed down as a result of a rigorous offseason program, will make his biggest maturity strides in 2012. The year is one Miller has designated as a reasonable target for his Major League debut. The organization isn't ready to add that expectation, but there is certainly a feeling that Miller is close.
"We're just going to watch," manager Mike Matheny said, "and pay close attention."
Miller, who has been deliberately placed alongside Adam Wainwright during these early spring workouts, could start the season in either the Double-A or Triple-A rotation. That decision, farm director John Vuch said, is still in the discussion phase.
While Miller could likely make the jump to Triple-A and immediately succeed, there could be a benefit in having him return, even if for only a brief stay, to Double-A Springfield, where Miller went 9-3 with a 2.70 ERA in 16 starts last year.
There, he could continue to work on mixing offspeed pitches around the use of his fastball. He could also tighten up other facets of the game that don't often draw the same attention as velocity or command -- components of the game like holding runners close or backing up bases.
"He's learning what he has to do to be successful," Vuch said. "Hitters are more selective at Double-A. I think that's why when he added the breaking pitch in, it allowed him to succeed."
If Miller, who was recently named the Cardinals' top prospect by MLB.com, were to begin the year in Springfield, he likely won't be there long.
"I've become a better pitcher," Miller said of his ongoing climb in the Minors. "I know how to get hitters out faster and not throw so many pitches in an inning. I'm just becoming more of a pitcher than a thrower. The offspeed [pitches] are coming along really well, and I can see that it's going to get better as it goes. I'm working [right now] around the best there is. I'm just taking it all in and getting ready for the season."
Aspirations of making his big league debut in 2012 are probably only partially in Miller's control. The Cardinals don't intend to rush the right-hander's ascension to the Majors simply as a reward for Minor League success.
Both need and timing will play a role in Miller's track this year, as there could be a scenario in which, though ready, Miller is held back from St. Louis because the team is in a pennant race or doesn't have an obvious candidate to boot from the rotation.
It's adversity that Miller likely doesn't think much about, but a reality that would again come back to that concept of maturity. And how Miller responds would be the clearest indication of growth.
"I know I'm so close to getting that [big league] opportunity," Miller said. "I really do feel like I'm close, and that makes me even more anxious to get ready for the season. You just never know what's going to happen. I think it's going to be a good year, and hopefully this is the season that I pitch in St. Louis."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.