For a movie about people who live to party, the set of the 2003 comedy classic “Old School” practiced what it preached.

In his new book, “Funny Business: The Old-School Wedding Crashers and Knocked-Up Virgins Who Changed Comedy Forever,” (G.P. Putnam’s Sons, Oct. 6), Matt Singer writes of how, between takes on “Old School,” Snoop Dogg would invite all of the attractive extras back to his trailer to hang out while he DJed. When an assistant came to get Snoop back on set to shoot his scene, he couldn’t get in the trailer to retrieve the star.

“There was a bouncer at the door not letting him inside,” Scot Armstrong, the film’s co-writer, recalls in the book.

The tome looks at how talents such as Judd Apatow, Will Ferrell, Ben Stiller, Seth Rogen, Todd Phillips, and others created a comedy movie boom of the aughts.

It was a time when Stiller could star in four movies in a single year that each grossed over $100 million domestic, as he did in 2004, and people would fill theaters to watch the antics of the actors known as “the Frat Pack.”

Despite that moniker, however, the group was often characterized by a nerdy outrageousness.

When Apatow was 12, he saw Steve Martin on the street and asked for an autograph, but Martin refused. The tween did not take the rejection lightly.

“Apatow wrote him a letter chastising him for treating his fans ‘like crap’ and threatening to send his address to (tour bus company) ‘Homes of the Stars’ if he didn’t apologize,” writes Singer. “A few months later, a copy of Martin’s book ‘Cruel Shoes’ arrived with the inscription, ‘Dear Judd. I’m sorry. I didn’t realize I was speaking to the Judd Apatow! Your friend, Steve Martin.’ ”

Apatow would need such bold audaciousness when it came time to make “The 40-Year-Old Virgin.”

While the movie eventually made Steve Carell a star, Universal Pictures was so disturbed by the first two days of filming that they shut down the production, summoning Carell and Apatow to an emergency meeting.

“The studio was horrified by endless footage of Carell riding a bicycle and walking down the street looking at posters of scantily clad women,” writes Singer.

“In the end, it had to do with a haircut. They thought Steve was too creepy-looking,” producer Clayton Townsend says in the book.

Universal executives interrogated Carell and Apatow with questions like, “He looks like a serial killer. Is he going to kill someone?” 

In the end, they gave Carell a haircut and cautiously allowed the shoot to continue — requiring another hairy situation.

For the film’s infamous chest-waxing scene, Carell actually got his chest waxed on camera so that Apatow could capture his true reactions. The director hired an experienced chest waxer to keep the pain to a minimum — or so he thought.

Apatow now suspects she exaggerated her resume because “she clearly didn’t know how to do it, and we almost ripped Steve’s nipples off during the shoot,” Apatow told Singer for the book. (The waxer actually was experienced and blamed the pain on Carell’s long chest hair, saying she “couldn’t find his nipple.”)

The excruciating realism made for comedy gold, and Carell’s agonized scream of “Noooooo Kelly Clarkson!” — a phrase scripted by cast member Rogen — became a hilarious meme.

Several of the era’s funniest movies faced dire challenges during filming.

“Tropic Thunder,” Stiller’s hit 2008 production about, ironically, the troubled set of a war movie, shot for 13 weeks in Hawaii and faced so many issues, including “constant rainstorms, bug bites, illnesses and injuries” that star Robert Downey Jr., who received an Oscar nomination for his role in the film, came to call it “Ben Stiller’s Comedy Death Camp.”

Around the same time, the Las Vegas shoot of 2009’s “The Hangover” was almost shut down by police when, while shooting one of the film’s more memorable scenes, they witnessed actor Ken Jeong jump out of a car trunk completely naked “in full view of the Mandalay Bay casino.”

That film both benefitted from and was almost marred by “Phillips’ taste for the dark side of human behavior,” writes Singer.

While the movie’s “kidnappings, beatings, car crashes, roofies, tiger maulings and tasings” drove it to $469 million at the box office worldwide — Phillips wisely refused a $6.5 million directing fee in exchange for a share of the profits that eventually earned him $70 million — he almost took it too far.

For a scene where stars Bradley Cooper, Ed Helms and Zach Galifianakis are tased, “Phillips tried to talk the actors into letting him tase them for real,” Singer writes.

“Warner Brothers lawyers stepped in, thank God,” Galifianakis says in the book.

After around a decade spent ruling the box office and the cultural zeitgeist, these male-driven comedies fell out of favor with the rise of superhero movies and streaming, along with the advancing age and salaries of the men who created them.

Given how the world has changed, the likelihood of a revival seems slim. Still, Apatow sees a timeless element to the appeal of these films.

“I look at [them] like Marx Brothers movies, [which were] comedies about these weird male friendships,” the director says in the book. “There’s a grand tradition — ‘Stripes,’ ‘Animal House’ — about the weird ways guys interact with each other because they’re idiots. They’re fun to watch.”