Naomi Watts has appeared opposite some of the biggest stars in film — Sean Penn, Viggo Mortensen, Leonardo DiCaprio, even King Kong. But none of those leading men prepared her for Bing, the 145-pound Great Dane she co-stars with in “The Friend,” a drama about heartbreak, art and a very large dog screening at this year’s Toronto Film Festival.

And getting used to Bing was a six-week process that involved more than just handing out treats. Bill Berloni, the veteran animal trainer tasked with preparing Bing for his close-up, insisted that Watts meet with the dog for 20 minutes twice a day so she could learn how to control the lumbering giant on the streets of New York City, where “The Friend” was shot.

“At first I thought, is this necessary?” Watts admits. “I’m an animal lover. I’m not put off by a little hair and drool.” She wondered if their meetings could be shortened to at least one a day, for 40 minutes, instead of breaking them up. But the thing about Great Danes is that their sheer size makes them, well, couch potatoes. “They get tired really quickly,” Watts says.

But that passivity belies their power. If Bing were to see a squirrel or another dog, or simply get spooked by a passing car or truck while filming a scene, Watts could be in trouble. “He’s so gentle, but if he were to run abruptly in one direction, your arm could pop out of its socket,” she says.