When is a romantic comedy not a romantic comedy? Universal Pictures' "The Break-Up," opening today, stars Vince Vaughn and Jennifer Aniston, two big names that naturally fit the romantic comedy mold. Directed by Peyton Reed, it has a colorful supporting cast of scene-stealers, a romcom musical number in which the cast sings a once-popular song and a muscular marketing campaign that makes it look like a successor to last year's "Wedding Crashers," Vaughn's raucous romantic comedy that grossed more than $200 million domestically.

But according to the filmmakers, it's not a romantic comedy. It's not even an anti-romantic comedy. Rather, they designed the movie to explore, in comedic and dramatic tones, one couple's breakup, when neither wants to move out of the condo they share. It's akin to Woody Allen or even Neil LaBute territory, except that in this case it was Vaughn himself who mapped out the terrain.

From the start, he wanted to keep the focus on the characters and make a realistic movie. "When you are doing a character-driven movie, it's not like you have a Death Star to blow up, and the good people win and the bad guys lose. Real life is more complicated than that," Vaughn says.

"If you explore a breakup between people, a lot of funny stuff happens, and you can home in on the comedy, but there is a painful side to it," executive producer Peter Billingsley says. "And it's unfair not to show that. It's a delicate act, I think. We spent a long time writing it."

Way before 2003's "Old School" launched Vaughn on his current comedy trajectory, Jeremy Garelick and Jay Lavender wrote a script called "The Golden Tux" with Vaughn in mind. It sold to Dimension in June 2002, and the two got a meeting with their prospective leading man. At the meeting, Vaughn said he had an idea for another movie about a couple that breaks up, and he wanted the duo to write it.

That project was set up as a pitch at Revolution but then fell apart. Garelick and Lavender, with Vaughn's blessing, wrote it as a spec. The first draft took 21⁄2 months to write. After that, with New York-based Garelick living in Vaughn's home in Los Angeles and the Hollywood-based Lavender showing up every day, "(We) rewrote and rewrote and rewrote," Garelick says. "We did improv with Vince. We'd act out every character to come up with the funniest and (most) meaningful stuff. And we'd talk story ad nauseam."

Studio execs liked the results: The script sold in a bidding war to Universal, netting the writers more than $2 million and a $10 million payday for Vaughn.

The result is a relationship dramedy that has more in common with such movies as "Annie Hall," "Paper Moon" or "Big Night," which the writers cite as their primary influences. Scenes turn from funny to serious, from dire to hilarious, striking every emotional note in between.

"You have to very carefully work out those beats," Billingsley says. "There are even dramatic scenes where you have jokes in the middle of them. And so you're walking a very difficult tonal line."

Comedy was taken out of the second half of the script, and the ending was redone and redone again -- a traditional Hollywood ending was created, then scrapped -- all with the goal of keeping it real.

"It was never intended to be a romantic comedy but rather something that captures the duality of life," Lavender says. "Sometimes the hard times are funny, sometimes they are painful. Vince said to us, 'Sadly, the person you learn the most from isn't always the one you get to end up with.' And I think we've all got those relationships."