Like so many leading men who appear ­larger than life on the big screen, the star of Steven Spielberg’s “War Horse” is, disappointingly, much shorter in person.

But whatever he lacks in stature, Finder’s Key more than makes up for in scene-stealing screen presence as Joey, the plucky thoroughbred who is wrested from his young master to serve both British and German armies during World War I.

Critical raves from Entertainment Weekly, The New York Times and Time magazine notwithstanding, “War Horse” — which galloped into theaters on Christmas Day — would be dead lame without the charismatic Joey (or, rather, the eight horses that play him as an adult horse).

The horse responsible for the lion’s share of Joey’s specialty work — rearing on command, running in an open field to a designated spot, lying motionless while ensnared in barbed wire (which is actually plastic) — is Finder’s Key, a sporty, 12-year-old California-bred gelding whose impeccably bred sire, Lindsey’s Roberto, ran 12 times but never won a race.

“Finder,” as he is mononymously billed, bears no battle scars from his own forgettable racing career, the evidence of which is confirmed by the fading vestige of a lip tattoo. The 15.2-hand gelding failed to hit the board in any of his four starts (all $2,500 maiden claiming races) at Los Alamitos Racecourse, a ­middling track south of Los Angeles that ­primarily runs quarter horse races.

Apparently, Los Alamitos is the Schwab’s Drugstore of the California racing circuit. In the summer of 2002, Finder was “discovered” by Hollywood horse trainer Rusty Hendrickson, who regularly visits the Los Alamitos backstretch looking for budding equine thespians. Hendrickson zeroed in on Finder while casting for “Seabiscuit” and bought him for $1,500 from Los Alamitos trainer Arthur “Curly” Ortiz on behalf of producers Kathleen Kennedy and Frank Marshall

“He was just impossible to train,” said Ortiz, who has sold “10, maybe 15” horses to Hollywood trainers over the years. “He would rear, and nobody wanted to get on him. He wouldn’t go to the track without a pony.”

But Hendrickson, who had already made an offer on an Appaloosa and a bay thoroughbred mare in Ortiz’s barn, saw star potential in Finder.

“He said he wouldn’t take the other two unless I sold him Finder,” Ortiz said, laughing.

On the “Seabiscuit” set, Finder was paired up with trainer Bobby Lovgren, a native of Johannesburg whose family runs one of South Africa’s premiere riding academies. Finder’s star turn comes early in “Seabiscuit,” where he can be seen biting and ripping a jockey’s shirt off. Lovgren was so impressed with the young gelding’s ability to follow cues that he bought Finder and brought him home to his ranch in Acton, some 45 miles north of Hollywood in the Antelope Valley.

Lovgren, 46, came to the U.S. in the 1980s to study under renowned horseman Glenn Randall Sr., who trained the 78 horses used for the riveting chariot race in “Ben Hur” (1959) as well as Roy Rogers’ faithful steed, Trigger. Lovgren’s credits include working with the zebras used in “Racing Stripes” (2005) and the Andalusians in “Legend of Zorro” (2005).

“He’s a typical thoroughbred,” Lovgren said of Finder. “He gets really wound up; he’s not a good riding horse. I do ride him, but he is not under any ­circumstance an actor’s ­(riding) horse.”

Lovgren and Finder recently returned from the Canadian set of “Mirror, Mirror,” which is slated for release in March and stars Lily Collins as Snow White and Julia Roberts as the Evil Queen. Finder’s passport also includes a stop in Mexico (for “Zorro”) and England (his only transatlantic flight to date) for “War Horse,” which was filmed entirely on location in the United Kingdom for three months in the summer of 2010.

In its inception, “War Horse” was a 1982 novel by Michael Morpurgo. In 2007, it became an award-winning stage play in London; this year, it came to Broadway and won five Tony Awards, including Best Play.

Many species served nobly in the Great War. Birds and dogs ferried messages, camels and mules lugged artillery … even cats were deployed to ambush the rats in the trenches. But the animal that contributed — and sacrificed — the most during World War I was the horse. It is estimated that between 4 and 8 million horses died on all sides, most from injuries sustained in battle, but others from starvation, disease or mistreatment.

Lovgren, who oversaw all animal training on “War Horse,” relied on Finder for some of the film’s most heartrending scenes, including one involving Joey and his friend, Topthorn.

Spoiler alert: As a dying Topthorn (actually a laid-back gelding named George) crumples to the ground, Joey (Finder) takes the reins and tries to pull him up.

Never before has the disclaimer “No animals were harmed in the filming of this motion picture” been so ­comforting as during the movie’s most terrifying sequence, Joey’s charge through “No Man’s Land,” which ends with the badly injured horse trapped in barbed wire. A life-sized, ­animatronic Joey was built for much of the harrowing barbed wire scene, though Finder was brought in for facial close-ups. The American Humane Association, which had a representative on the set at all times, gave “War Horse” its highest grade: Outstanding.

Finder’s forte is “liberty work,” which involves Lovgren directing him with no restraints or tethers, often in large, unfenced areas. “I know my boundaries with him,” Lovgren said, laughing. “It’s really all about teaching him to have confidence in me. He could certainly run away if he wanted to.” 

The breathtaking scenes of Joey running free in the lush green fields of Devon were the most difficult performances for Lovgren to coax from Finder. “Here at home, all we have are dirt paddocks,” Lovgren said. “When he saw all that grass, he just wanted to eat it. That tortured me a bit. The filmmakers would say, ‘OK, we want him standing still and looking at us.’ Good luck!”

If a horse’s worth is gauged by how much he bankrolled on the racetrack, Finder would be worth $500. “He’s by far the most intelligent horse I’ve ever worked with,” Lovgren said. “He seems to enjoy showing off when people are watching. I don’t know how he knows it. He truly is Mr. Hollywood. You can’t put a price on him.”

The day after Christmas, Ortiz took his girlfriend, Lisa, to see “War Horse” at a large multiplex in Ontario, Calif. During the cavalry scene, Ortiz thought he spotted his old friend running neck and neck with Topthorn.

“The black horse pulled away from him,” Ortiz said, “and Lisa leaned over and said, ‘He still can’t run.’”

Contact Stephanie Diaz at MediaPlan88@aol.com.