Seth Rogen readily admits he’s a reluctant dieter. But is his public avowal to turn his lovable, Teddy Bear body into a lean, mean fighting machine for his lead in The Green Hornet coming undone? Spies say the Vancouver actor may be falling off the broiled fish and fruit wagon. Rogen, 28, a former regular at the In-N-Out burger joint near his L.A home, has been visiting the drive-through again.

“He’s been back in lately ordering the double/double (hamburger) with onions and fries,” said a well-placed source. When Rogen started his regime after Christmas, and lost at least 30 pounds through diet and exercise, the plan was for him to don the skin tight hero’s outfit for The Green Hornet this summer.

“I’m selling out, guys,” he apologized to fans of his movies such as Knocked Up, The 40-Year-Old Virgin, Superbad and most recently, Funny People, roles where he plays the attractive and charming, but overweight schlub. “(The diet) is for The Green Hornet. The only reason. There’s no other reason.”

But The Green Hornet, which he co-wrote, suddenly became a lot bigger and more expensive with the signing on of Cameron Diaz as his love interest and the ongoing search for the villain — rumoured at one point to be Nicholas Cage — caused the shooting to be postponed by months. (Shooting still hasn’t begun but sets are being built and Sony Pictures has committed to a June, 2010 release date).

“He didn’t think he’d have to diet this long and it’s got to be hard for him,” said Hollywood paparazzo Richard Terry, who follows Rogen regularly. “He shrunk to almost nothing for a while there and now he’s seeming a little bit heavier, I’m seeing that.”

As a consummate professional, I’m sure Rogen will be doing extra sit-ups to make up for any diet slips before he shoots love scenes with the gorgeous Diaz.