Like most NBA teams, the Celtics conduct their practices behind a veil of secrecy. Actually, they do their business behind a curtain, which goes down over the viewing area from the media room’s perch above their practice court in Waltham.

Typically, the curtain will rise for the last few minutes, when the assembled press will jockey for position to see who is on the court, or more importantly, who isn’t. If we’re lucky, we might get a glimpse of the end of a drill, but more often than not, all we get to see are players shooting free throws.

Either way, the glass window provides a mostly soundproof environment for the team to work. The only audible noises are the squeaks of sneakers on the court and the occasional scream, generally assumed to belong to Kevin Garnett. The Celtics had an open practice Tuesday for sponsors, which the media also was allowed to watch, and it allowed a rare glimpse into the team’s daily activities.

The first thing that stands out is the absence of banter or fooling around. The Celtics have developed a reputation for being loose off the court, thanks mostly to the arrival of the Big Maestro Shaquille O’Neal, but when it’s time to do their job, they work.

Then there is Doc Rivers, who cuts an active, athletic image in shorts and workout shirt. Part of Rivers’ genius is he still thinks like a player and he loves to encourage the reserves when they make a nice play. It’s partly to acknowledge them, but it’s also to subtly tweak his veterans in the starting lineup.

“The first unit knows that if they up their pressure defensively they can take the second unit out of anything we want to run, and they do that on purpose,” Rivers said. “They should do that.”

Rivers never blew the whistle that hung around his neck, but he didn’t have to because everyone knows he’s in charge. In his hand is an ever-present green sheet of paper with that day’s practice plan. That’s something he picked up from Pat Riley and he’s borrowed here and there from the coaches he’s played for.

“Riles, with the green sheet and the notes,” Rivers tells me after practice. “Larry [Brown] with being meticulous and stopping it and breaking it down. I tried not to stop it today. You almost had to … and then Mike Fratello, just the organization.”

The Celtics have practiced only sporadically over the last month or so. Part of that is the schedule — they never practice after back-to-backs — and part of that is due to the injuries. For the first time in weeks, Rivers actually had 12 players available, and even Kendrick Perkins went through the non-contact portion.

Considering the lack of time, and the public audience, Rivers expected this practice to be sloppy, and it was to a certain extent.

“Listen,” Rivers said to his team. “We’re playing hard, but we’re fouling a lot and we’re turning the ball over a lot.”

That is about as stern as it gets. There are no histrionics, just the occasional forceful reminder when someone goes the wrong way on a set. There is, however, a lot of teaching. Rookie guard Avery Bradley receives the bulk of the tutorials, which makes sense because with Rajon Rondo and Delonte West out of the lineup with injuries, Bradley is being force-fed minutes.

“I think there’s far more teaching in NBA practices than people realize and there’s far more structure and far more detail,” Rivers said. “I always say we have to teach the college kids when they come in the league. They don’t know as much as you think they would know and most of what they learn is probably in this league. They get the credit back in college and that’s fine with us as long as we win.”

While Rivers absorbed the lessons from his playing days, he also does things his way. One thing he stresses with his coaches is he wants them to be heard, and lead assistant Lawrence Frank is an active part of the practice.

“As a player I always thought the players should hear the assistants’ voices more,” Rivers said. “I always wondered why the head coach talked the whole freaking practice. I always thought, ‘Man, if I’m ever a coach I’m going to make sure my coaches are talking.’ Because when you’re in a game and you need an assistant to go say something to a guy and if they haven’t heard him all week, they’re not going to listen to him now.”

Aside from Rivers and Frank, the loudest voice belongs to Garnett, who believes that communication is the single most important element of playing team defense in the NBA. True to form, Garnett can be heard on every single possession.

With Nate Robinson isolated on defense at the top of the key and a screen coming from the man he’s defending, Garnett takes over. He doesn’t so much yell as make himself heard. “By yourself, Nate. By yourself, Nate.” Then when the screen comes, “It’s hot, Nate.”

Garnett continues to do this every single time, over and over. This is a small but important part of what he is trying to accomplish during the workout, and in doing so he is also setting an example for the other big men. (Garnett also talks to himself as much as he talks to other players. After setting a screen that freed Ray Allen for a jumper, he blurted to one in particular, “Nice pick, Kev.”)
Garnett is a fantastic practice player. He is active right from the start and he works himself into a quick sweat. Oddly enough, that presents something of a problem for Rivers.

“Almost too good,” the coach said. “That’s Kevin every day. The problem we’re having right now is not having enough guys to sub Kevin in practice and that actually hurts because we need Kevin for the whole year. I told him before practice, I’d like you to not do a lot of the active stuff if you can control yourself, and obviously he couldn’t. That’s just Kevin.”

This ultimately is what sets the Celtics apart and what truly stood out from Tuesday’s practice. Essentially, they’re basketball geeks. Yes, they’re professionals and yes, this is a job, but it makes the work easier when you actually enjoy what you’re doing.