Gabriela Cowperthwaite, who directed the groundbreaking documentary “Blackfish,” makes her feature film debut with “Megan Leavey,” the true story of a female marine who fought to be reunited with her K9 partner, Rex.

After being assigned the most aggressive dog in the K9 unit, Megan and Rex bonded and became an impressive team who saved hundreds of lives by sniffing out bombs in Iraq. After both were injured in the line of duty, Megan was sent home to recuperate, but Rex, who was deemed “unadoptable” because of his often aggressive nature, was re-deployed with a new handler. Megan began a campaign to bring Rex home, finally succeeding.

The film stars Kate Mara as Megan and German Shepherd named Varco (also making his feature film debut) as Rex. The role required Varco to bare his teeth and growl when he first meets Megan and not to flinch with the sounds of explosions and gunfire on the set.

Cowperthwaite talked to Moviepaws about Varco, the real Megan’s reaction to the dog who played her beloved Rex, and why bringing K9 soldiers home is so important to her.

“Megan Leavey” opens June 9.

Moviepaws: Did you anticipate what an enormous impact you’d have with “Blackfish”?
Cowperthwaite: No, never. I think if I would have had any idea what it was going to do, I would have run for the hills. I was not prepared for it striking a nerve like it did.

Why did you want to make this movie?
It was the same reason I wanted to do “Blackfish.” I had questions about how does a female Marine come up in that world? What is the K9 unit all about? How nuanced is that relationship? For me, it’s always story. If you can effectively tell a story, then can you back people into caring about something, rather than telling them to care about something? Can you crack open empathy and see something that in a different way or a different light? And then if they go do something about it, like start a petition or look up “war dogs” and what it means to bring a dog home.

Tell me about Varco, the dog who plays Rex.
He’s so lovely. He’s just an amazing animal. He’s got this wolf body, but this emotive face. He looks very much like the real Rex. He’s a tremendous animal who loves being loved. Loves his treats. I never want an animal to tons of takes, so for us, [but I knew he would] deliver it. And Kate, who is such an animal person as well, will deliver that gold. It’s up to us to be rolling. And us to be able to be spry on our feet to capture [the moment] and then [let him] go and play.

How do you get him to act vicious when the scene called for it?
We have a squirrel on the end of a stick and all he wants to do is chase it. That’s the barking stuff, believe it or not.

So the scenes where he seems aggressive is just excitement?
It’s totally excitement. He wants to run after that little fuzzy thing. It’s hilarious. You cannot bring that thing in anywhere near him, without him going nuts. Our first priority is always him. He’s the one who didn’t choose to be here. All the rest of us did. Let’s just make sure we respect that and if there’s a moment when he’s uncomfortable, we’re done. It was kind of a concern of mine, how do you make him aggressive? How do you make a dog aggressive without screwing with that animal? To realize it was a fake squirrel on a stick? It was like, “Okay, good!” But even that, you didn’t want to do too much. Because you’re not letting him get it, so even that level of agitation, you just don’t want that. You don’t want to give it to him. Of course, you do eventually give it to him.

But you get your take in the meantime.
Yes, you get your take. And you know what, if we missed it, that’s kind of too bad. I like the authentic take even if it’s not perfect.

What was Kate and Varco’s working relationship like?
He’s so big. They’re almost the same size. She understood that it would not fit for either of them to suddenly start cuddling. He’s not that kind of dog. He does like the affection, but he’s not one of those dogs that immediately comes over. We had other dogs on set who were like that, who were really love motivated. He was not like that. She was really, really respectful of him. But was beautiful that eventually, having spent so much time together, there was a moment when they’re in combat and there’s blanks being fired and squibs in this combat scene and she had a moment where she was nervous. She was like, “I have to run across this field with him and he’s so big and if he starts running really fast, he will drag me.” [Marines are tethered to their bomb-sniffing dogs, as were Kate and Varco on the set]. She started feeling nervous and he stopped and looked up and licked her face. It was just this beautiful thing. This is like a dog who doesn’t do that. She was like, “Now we’re bonded.” And no one knew it because there was all this combat going on.

How tough is it to get an animal to not bolt when he hears explosions?
You do a little bit from the background. A lot of this stuff is not right in his face. If you get him acclimated a little bit to it, he doesn’t even turn. He doesn’t even look. Even [those scenes], I was like, “Once we got it, we’re done. We’re not going to do eight takes.”

In the press notes, it says there was a moment when Varco ran off with Kate.
Yeah, and she was kind of stumbling behind him. That was right around when she was kind of nervous and they ran and she kind of twisted her ankle, but kept running. And that was it, we just cut. If he starts running nobody is going to be able to handle that. He’s so vast!

And she’s tethered, which is the way they really do it?

Yeah, she’s tethered. You can’t call him back, it’s not like a choke collar. It’s up to him. She needs to have control, and he runs like the wind.

Can you talk about Chico, Ramon Rodriguez’s dog in the movie? The press notes say he was like Paul Newman because he was making everyone laugh.
That’s funny! They’re such amazing dogs, the Belgians. They’re so strong. You always think the shepherds are the strongest, and then you watch these Belgians and how they run and how they jump. They jump with their whole bodies, whereas the German Shepherds will just use their heads. Chico was just so sweet and so into Ramon. They spent all this time bonding even before Ramon shot. He just wanted to hang out with his dog.

Was Chico his real name?
Yeah, they have him a new name in the script, and Ramon was like, “No, I’m using his name.”

Was it hard to have Varco respond to being called “Rex”?
No, actually. I think a lot of times when Kate said “Rex,” that wasn’t what made him do the thing, it would be the squirrel on the stick. Or it’s the treat hidden behind the pillow.

Where is Varco now?
I believe he belongs to the trainer.

Did the real Megan meet him or visit the set?

She did visit the set, but it was early on, in the boot camp section, before Kate meets Varco. But when she saw Varco in pictures and in the film, she broke down. She was like, “That’s Rex. He looks exactly like him.” She had a real emotional reaction to seeing him.

I understand the movie is being screened free for military families.
Yeah, it’s important to us that they see it. It’s important to me how these people come home and how these animals come home and how to train us, as civilians, to give them what they need. The sacrifice of these animals is unimaginable. What they’ve done in country to save hundreds of lives, and the idea that they’re not given a safe home afterwards is kind of unconscionable. If nothing else, hopefully this film can lead people to feel that. That goes for human veterans and animal veterans. What is it what we need to do to give these people when they come back broken? You can’t imagine that they can’t be a little bit broken.

Do you think it’s improving, that people are more aware of this issue of K9 soldiers?

Slightly more. I think the military’s very aware of it. I don’t know if the civilian world really understands the K9 unit and what it is that we’re asking these dogs to do. Megan got to come home and get out, but Rex had to be redeployed. That’s part of her plight, just the pain that she’s going through on a daily basis. It’s not only not feeling whole because her animal’s not with her, but knowing that he could be killed when he saved her life not only in physical ways, but also in emotional ways. Clinton did Robbie’s Law, which really meant taking care of these animals when they come home, but you hear stories of how they’re left in country at times, so it’s not an airtight policy.