Jarhead co-stars Jake Gyllenhaal and Peter Sarsgaard fought for real on the set of the army movie, after a fight scene left the latter with a broken rib.

Sarsgaard admits he punched Gyllenhaal “as hard as he could in the windpipe” during a bout of “macho crap”, intensified by their real life relationship – Sarsgaard is married to Gyllenhaal’s sister Maggie.

The fight took place during a scene that required a mob of soldiers, including Gyllenhaal, to brand Sarsgaard’s character.

Sarsgaard was hurt during the first take – he later discovered he had suffered a broken rib – and he snapped when director Sam Mendes ordered further takes.

The star tells GQ magazine, “When they came on take two, they looked like they were coming just as hard, so I hit Jake as hard as I could in the windpipe and kind of claimed it was an accident and stuff. And they came for a third time, and it was really hard again, and I just… I lost it.”

The pair didn’t speak for 10 days – but he insists the bust-up eventually strengthened their friendship.

Sarsgaard adds, “The nice thing is, for Jake, he doesn’t see having any argument as something that ruins a relationship. He sees it as something that could potentially help one.

“I’m extremely close to him. He’s basically my brother.”