CBS newsmag “60 Minutes” is celebrating its 30th year, and in that time, no one from the outside has ever gotten in there and documented what makes the show tick, tick, tick …

Until now.

The PBS series “American Masters” will present “Don Hewitt: 90 Minutes on 60 Minutes” on May 13, the result of producer/director Susan Steinberg’s eight-week stint turning the tables on the “60” crew.

It took “American Masters” exec producer Susan Lacy more than three years to convince her friend Hewitt to commit to the process, but “suddenly it was just the right time for him, with his 75th birthday, his 50th anniversary of being at CBS, and celebrating 30 years of ’60 Minutes,'” Lacy told Daily Variety.

To be deemed an American Master, Hewitt had to agree to “total access,” and he did just that – resulting in intimate behind-the-scenes footage of the show’s daily grind and candid chats with Hewitt, his cadre of star correspondents, and with his lesser-known and apparently frightfully overworked team of producers.

The doc takes the viewer through the making of a couple of “60 Minutes’ segments, one about Bobby Kennedy’s kids and one about “gullible” Americans in Ecuadorian jails on drug charges. The mix of ego, palpable tension, and careful attention to every last detail — all orchestrated by Hewitt — are quite visible.

Steinberg’s camera is the first ever to be allowed inside the legendary “screening room,” where Hewitt views and comments on every single report created by staffers. Even stars like Leslie Stahl and Mike Wallace get nervous before being subjected to Hewitt’s tough criticisms.

In fact, Stahl wouldn’t allow Steinberg to film her screening session, and while Wallace did, the footage has him exiting the room peeved, smirking “Can we leave? Thank you, master.”

Staffers talk openly about the nixed 1995 tobacco story (featuring whistle-blower Jeffrey Wigand discussing his former employer, tobacco giant Brown and Williamson), that shook the show’s foundation.

And Steinberg even managed to catch Hewitt playing the game he loves to hate: ratings. Hewitt is a loud critic of ratings-driven news, but is caught twice on camera delving into the minutiae of overnights and how his competitors are doing numbers-wise.

While there is much made of the healthy competition that keeps the show edgy, one of the more interesting points that didn’t make it into “90 on 60” is how tough the criticism can be on newcomers.

Part-time correspondents Christiane Amanpour and Bob Simon were strangely quiet on the much reported-on hazing phenomenon. But Steinberg says she regretted having to cut Steve Kroft being “quite open about having to decipher the unwritten rules guiding the show, and how when you first start, you don’t want to do a great story because people are out to get you.”

Speaking to Daily Variety several days before Hewitt and company had even seen the doc, Steinberg said she had tried to be fair, but “not deliver a valentine.” She was nervous at the prospect of sitting in a room with Hewitt as he viewed her work for the first time.

On Monday, Hewitt and his staff filed into the famed screening room, with Steinberg in tow. They watched, they laughed, they reflected.

“How could I not be happy with this,” Hewitt later told Daily Variety. “She got it.”

And there was probably no tougher room in the world to please.