Actor Jon Hamm was touched to discover the "ridiculous tradition" at the centre of his new action comedy Tag was based on a real-life game between friends.

The Mad Men star features alongside Jeremy Renner, Ed Helms, Isla Fisher, Jake Johnson, and Annabelle Wallis in the film, about a group of highly-competitive childhood pals who have dedicated one month a year to play a no-holds-barred game of tag for almost three decades.

Hamm and Helms admit the premise behind the movie seemed a little silly at first glance, until they learned all about the true story, which was documented in The Wall Street Journal in 2013.

The Washington natives their characters are based on got the opportunity to visit the cast and crew on set during production, and Hamm reveals even they can't believe their story has become Hollywood fodder.

"We had such a good time making this movie and... I thought we did a really nice honouring of these guys because we got to meet these guys... and they were just giddy at the fact that this ridiculous tradition that they've had going on for over three decades, has been made into a movie," Jon explained in a joint interview on breakfast show Good Morning America.

"They were just like, 'You guys are doing this? Really? We started this kinda as a weird thing and didn't know when to stop... but now it's a movie!'"

Hamm admits the unique bond is partly why he and Ed signed on to the shoot: "When you meet these actual guys, you realise that it's the thing that kinda has cemented their friendship for so long, and it's really kind of lovely," he smiled. "It's just a really nice warm-hearted story about friends, and that's what I think attracted both of us to the project."

"I don't think you can make this movie unless it's real, 'cause it's too absurd of a premise...!" confessed Helms. "I mean, if you told me that this was an idea for a movie, I'd be like, 'No one will believe it. It's ridiculous!'"