In case you’ve somehow missed the billboards plastered on every piece of available real estate in America, Judd Apatow’s Funny People, on which he serves as a triple-threat writer-director-producer, is being released this Friday, July 31. Apatow has been behind some of the most successful comedies in the past five years, including The 40 Year Old Virgin and Knocked Up. So what’s the secret to this comedy guru’s success? It’s the actors he has chosen to work with time and time again—gifted comedians in their own right who appreciate the improvisational style of Apatow’s films. One might call them Funny People. And when Apatow finds an actor he clicks with, you can bet you’ll see them collaborate on more than one occasion. Here, then, are 10 actors Apatow seems to love working with—and for good reason.

Seth Rogen
As a lead character in two of Apatow’s short-lived—but much-beloved—TV series, “Freaks and Geeks” and “Undeclared,” Apatow and Rogen go way back. Rogen got progressively larger roles in a few of Apatow’s films before finally landing a lead role in Knocked Up. Speaking on Funny People, Rogen told MovieMaker that it’s a film that is “completely unlike our other movies” and “is a step forward for us all.”

Paul Rudd
We had seen him long before he started appearing in Apatow films as Phoebe’s husband on “Friends,” Cher’s stepbrother in Clueless and Andy in the outrageous Wet Hot American Summer. But Apatow’s compulsion for employing first-class comedic talent has caused him to put Rudd in a whole slew of his films, beginning with Anchorman. Rudd has appeared in more than a handful of Apatow films, even more non-Apatows and recently earned himself a screenwriting credit on Role Models.

Jonah Hill
Hill’s first Apatow film was The 40 Year Old Virgin, where the 25-year-old Los Angeles native tried to buy a pair of silver heels at a store called “We Sell Your Stuff on Ebay.” Since then, he attained a leading role in Superbad, where his hilarious rants were somewhat reminiscent of Vince Vaughn in Wedding Crashers. He has since had various roles in several other Apatow films, including Forgetting Sarah Marshall and Funny People, and will have another lead in Get Him to the Greek.

Leslie Mann
This is one actress Apatow undoubtedly loves. She is, after all, his wife. The two met on the set of the Apatow-produced The Cable Guy, and Mann has since appeared in more than a dozen films. The Apatows have made it a family affair, with their two daughters—Maude and Iris—appearing in Knocked Up and Funny People alongside their mom.

Steve Carell
After initially starring as the lovable idiot Brick Tamland in Anchorman, Carell went on to become The 40 Year Old Virgin. He even makes a cameo as himself in Knocked Up, where he uncomfortably walks away from an interview with a very hormonal Katherine Heigl. He currently stars in the hit TV show “The Office,” in addition to many film roles including Horton Hears a Who! and Get Smart. This Golden Globe winner will also be appearing in the 2010 film Date Night with fellow actor-writer-producer Tina Fey.

Russell Brand
Apatow doesn’t just love Brand, he also seems to love his Forgetting Sarah Marshall character, Aldous Snow, who will be the main character in the upcoming Get Him to the Greek (which is, like Sarah Marshall, being produced by Apatow). It may seem as if Apatow discovered Brand, but as the British comedian often tells Americans in his stand-up: He’s famous in England, and unfortunately, “without fame, this haircut just looks like mental illness.”

Bill Hader
Hader has been an “SNL” cast member for four years now. Some of his recurring characters on the show include Nitro in the ongoing Digital Short Laser Cats, real-life former Governor Eliot Spitzer and a character called DJ Frontal Assault. He first appeared in an Apatow film with his role as Heigl’s E! editor in Knocked Up. He has since appeared in Superbad, where he and Rogen play two crazy, rule-breaking cops. When asked to return to the Apatow crew with Forgetting Sarah Marshall, he was eager to do so because he enjoys the collaborative nature of the films, as evidenced by his supporting roles in five Apatow films.

Will Ferrell
This former “SNL” cast member had many acting credits under his belt before he and Apatow started collaborating, but when the two got together they struck comedy gold. Anchorman, starring Ferrell, was the first film in Apatow’s recent series of very successful films, and was the breakthrough movie for many actors previously mentioned. In addition, Apatow helped pioneer the Ferrell-as-a-crazed-athlete genre, producing both Kicking & Screaming and Talladega Nights. Apatow also seems to enjoy the Ferrell/John C. Reilly combo (as we all do), with a producing credit for both Talladega Nights and Step Brothers.

Jason Segel
Segel is another former “Freaks and Geeks” and “Undeclared” cast member who was not left behind. After playing one of Seth Rogen’s stoner buddies in Knocked Up, Segel not only landed himself a lead role in Forgetting Sarah Marshall, he wrote it. He has since appeared in the non-Apatow film I Love You, Man alongside Rudd, in addition to his leading role on the hit sitcom “How I Met your Mother.”

Adam Sandler
Though they haven’t worked together as much as some of the other Apatow crew members, Judd and Sandler have a history. Before either of their careers really took off, the two were roommates in their early 20s. Sandler is also a former “SNL” cast member who has had a lot of success as an actor/comedian for well over a decade, including starring in Big Daddy with Mann. He also starred in the Apatow-written film, You Don’t Mess with the Zohan, playing the titular sex-crazed, Israeli hairdresser, and is the lead in Funny People.