TORONTO – Viggo Mortensen follows a classic cowboy code in the Western “Appaloosa”: Speak softly and carry a big honkin’ gun.

As an Old West lawman, Mortensen packs a booming eight-gauge shotgun in “Appaloosa,” which reteams him with “A History of Violence” co-star Ed Harris, who also directed and co-wrote the Western.

Fifty inches long and weighing 11 pounds, the eight-gauge initially was a turnoff for Mortensen when shooting began on “Appaloosa.”

“When I first had it, I said, ‘Do you really need it to be an eight-gauge, Ed?'” Mortensen, 49, said in an interview at the Toronto International Film Festival, where “Appaloosa” premiered in advance of its theatrical release Friday.

“It’s not that manageable, it’s not going to be accurate at much distance. I said, ‘I’m not going to shoot that thing off a horse, because I’d get blown off the horse, realistically.'”

After a day or two, Mortensen started looking at the eight-gauge as an ally, a handy reminder to bad guys that the law can always outgun them.

At that point, Mortensen started lobbying for a bigger role for the gun, which sent dogs and horses running the first time he shot it outdoors and which rattled the windows and floorboards when he test-fired inside a saloon.

“I said, ‘Ed, you know, I think I should even have it indoors. Even if I’m being friendly or if I were buying some fruit or getting a haircut, I always have it with me,'” Mortensen said. “It’s just an intimidation thing, just like our larger-than-the-other-horses horses are. So once you’ve seen it fired, you don’t need to see it being shot again.”

“Viggo handles props great, and he loves detail, so that thing was his baby,” said Harris, who compared the eight-gauge to an elephant gun.

Co-star Renee Zellweger said Mortensen and his eight-gauge became inseparable.

“He had it everywhere, all day, every day,” Zellweger said. “There’s a scene that’s not in the film where he carries my luggage out of the diner, the cafe, and he had to figure out a way to open the door, grab the suitcases, close the door, and all the while hold that gun.”

Adapted from Robert B. Parker’s novel, the film is the story of two old trail buddies, Virgil Cole (Harris) and Everett Hitch (Mortensen), itinerant lawmen who sign on to clean up the town of Appaloosa, where a murderous rancher (Jeremy Irons) runs the show.

Cole and Hitch’s efforts are complicated by the arrival of widow Allie French (Zellweger), who begins a capricious romance with Cole.

Harris pitched the story to Mortensen while they were at the Toronto festival in 2005 to promote “A History of Violence,” the first of a number of smaller projects Mortensen took on after completing “The Lord of the Rings” trilogy and the epic “Hidalgo.”

They had been on opposite sides of the law in “A History of Violence,” Mortensen a diner owner trying to protect his family and Harris a savage gangster trying to suck Mortensen back into his violent old ways.

“I just really enjoyed working with him,” Harris said. “He’s a really decent guy, a wonderful actor, a great-looking actor. I thought the two of us could capture this kind of unspoken love, appreciation that these guys have for each other. And his sense of humor. He’s got kind of a weird sense of humor I like.”

A best-actor Academy Award nominee for 2007’s “Eastern Promises,” Mortensen follows “Appaloosa” with two more films this fall. In “The Road,” adapted from Cormac McCarthy’s novel, Mortensen plays a man struggling to survive with his young son in a bleak post-apocalyptic landscape.

In “Good,” which also played the Toronto festival, Mortensen stars as a novelist, professor and all-around decent man who is gradually lured into Nazi complicity in 1930s and ’40s Germany.

“It’s about trying to make the right choices, but then you go along a little, then a little more, and then you try to justify what you’ve gone along with,” Mortensen said.