Strays is filled with doggone great actors.

The R-rated talking dog comedy — in theaters August 18 — from Universal Studios features over 75 canine actors, according to Mark Forbes, the head dog trainer on the film.

Strays tells the story of Reggie (voiced by Will Ferrell), an adorable terrier abandoned by his unloving owner. Oblivious to the better treatment he deserves, Reggie decides to find his way back home to his negligent pet parent and is joined by three other canines on his journey: Boston terrier Bug (voiced by Jamie Foxx), Elsa the Australian shepherd (voiced by Isla Fisher), and Hunter the Great Dane (voiced by Randall Park).

And while A-list celebrities speak for the pups in the movie, it was up to the canine stars to do most of the on-screen acting.

"I really wanted to lean on naturalistic dog behavior. Anybody who owns a dog or a cat knows how their body language tells you so much. And so, to that end, I really wanted to get the behavior out of the dogs using a team of trainers and not by not relying too much on CGI," Strays' director Josh Greenbaum tells PEOPLE.

"If a dog was confused, instead of animating the eyebrows and furrowing them the way you might in either a kid's movie, they cock their head. That's what they do when they're confused," he adds.

Greenbaum looked to animal movie classics from his childhood for inspiration.

"Whenever I was sick growing up, I remember watching Homeward Bound and Milo and Otis," he shares. "Those were certainly big touchstones. We wanted to take those movies and flip them on their heads but also celebrate and honor those movies because they are great."

After hearing about Strays, Forbes was instantly on board to help bring the vision to life.

"For years, I have been saying, 'Somebody needs to make an R-rated talking dog movie,'" the animal trainer says.

The first step in making Strays' vision a reality was finding the ideal canine actors for the job.

Three of Strays's four main dog actors "started fresh," says Forbes, meaning the pooches didn't have any acting experience before the film.

"There's no shortcut, so it takes us four to five months of working with the dog to sort of get them ready for filming," he says.

"A lot of things that may make a dog not a great pet make them a great candidate for us. Dogs with too much energy that are a handful as a pet at home are great for us because we can take all that energy and channel it. You can always channel the energy. It's trying to get it out of a dog that doesn't really have it that's more difficult," Forbes says.

Unless the dog actor is lent to the trainers by a pet owner, "we always adopt them even before we start a shoot," Forbes says. "Once we take them on, we take them on forever."

After the furry actors for Strays were selected, Forbes and the other trainers broke down the film's script into trainable actions for the dogs and went to work helping the pets master these commands.

Forbes has worked on numerous films, but this raunchy comedy presented some unique first-time challenges for the trainer.

"There were so many things that I had never done before in this film. In fact, one night, it was about 2:00 in the morning, and we were shooting dogs humping lawn ornaments," he says.

There were also more technical issues to consider as well.

"Talking dog movies are about eye lines. They're talking like humans, so people expect them to act somewhat anthropomorphic and look at the person they're talking to. For us, it becomes about getting that eye line right all the time. So when the Great Dane in the movie looks at Reggie, he has to look down at him. So that's a trainer laying on the ground with a little bait stick behind Reggie trying to get that look," Forbes says.

What matters most to Forbes is that his canine partners are having a good time.

"I can't give them a bigger paycheck. If they're not having fun, they're not doing it. So a big part of our job is making sure they're having fun every day, so they're looking forward to coming to set," he says.

This upbeat, pet-friendly vibe on set also benefitted the humans behind Strays.

"It was a warm, wonderful set to be on," Greenbaum says.

"They say dogs lower your stress level, which you could feel on set. Having them around, it calmed everybody. Everybody was petting dogs. It just created a very nice environment," the director, who adopted one of the puppies featured in the film, adds.

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories

The canine stars of Strays got to see the result of their work this week at a special screening of the film at Universal CityWalk in California. The pups walked the grass carpet and met adoring fans before taking in the movie.

Strays is now in theaters nationwide.