Exclusive: Gag reel shows goofy 'Neighbors'

Neighbors was so funny that even the stars had trouble keeping a straight face during the film's production.

The Nicholas Stoller-directed comedy featured Seth Rogen, Rose Byrne, Zac Efron and Dave Franco. It was a major player in the spring box office, taking in over $50 million in its May opening weekend.

This gag reel from Tuesday's Blu-Ray/DVD release shows the mayhem taking place behind-the-scenes.

"It was easily the most laid-back and fun shoot I have ever had," Stoller says. "And there was a lot of breaking the scene (with laughter). It happened all the time."

This unintended laughter was great and showed that the chemistry was working. But Stoller says there was one scene with Rogen and Byrne where the two actors couldn't pull it off because they kept laughing and spoiling the footage.

"Finally I had to be like, 'Okay, guys seriously,' " Stoller says. "I had to do that voice. It was like, 'This is funny, but we have to get this shot.' "

Another scene featured fraternity brothers (Franco and Efron) hosting a Robert De Niro party next to the new parents played by Rogen and Byrne. Franco learned how to pull off a killer Robert De Niro imitation.

"(Franco) spent weeks figuring it out. He's hyper professional pretending to be mellow," Stoller says. "He was having trouble with it. But the Sunday before we shot, he just looked in the mirror and kind of squished his face into the De Niro face and it clicked into place."

The problem was, the contorted face was painful during a long set day.

"Dave was doing the face off-camera so much that he strained his face muscles," Stoller says. "So when we turned around the camera on him, suddenly he couldn't do it because his face was hurting."

As you see on this clip, the hilarity ensues.

"They kept cracking up," Stoller says. "But there was a lot of amazement that Dave was so good at it."

Franco eventually nailed the impersonation for the scene.

Even stars like Ike Barinholtz (as Jimmy) and Lisa Kudrow (as the college administrator) had their own personal issues.