That trailer you can't stop watching used it. One of Disney's most memorable songs did it. And it's in almost every movie you see on TV or on an airplane.

You've probably never heard of voice matching before, but it's one of Hollywood's oldest and most useful tricks.

Like stunt doubles or digital retouching, voice matching — also known as "voice double" or "soundalike" — is a tool the movie business uses to conjure the fantasy we imagine in our heads. It's a process in which voiceover artists are hired in postproduction to come in and double for the voice of a star. And if the voice match is done right, you’ll never know that a line of dialogue actually came from someone other than the actor you see on-screen.

It often happens when an actor is already working on another movie and can't come in to do the ADR session (additional dialogue replacement), which takes place months after filming. And there are some stars who simply hate doing ADR, even adding clauses refusing to do it in their contracts. There's good reason for that: ADR is quite challenging.

When a movie or TV show is in postproduction, all sound has to be mixed for the footage that's being used in editing. If dialogue can't be heard because of noise on the set or a mic malfunction, the actor must come in for ADR sessions to rerecord the dialogue. (Actors are also asked to come in to do "clean versions" of movies, dubbing over curse words with words that will be suitable for TV or airplanes.)

But the lines must be delivered with the exact tone that was used on the set. If you were out of breath then, you have to sound that way again.

"There are those actors who hate to do ADR," supervising sound editor/rerecording mixer Michael J. Fox told Business Insider. Fox has been overseeing voice matching since the late 1990s. "They are far removed from what they did on-set, there's no one to play off of, you can't get back in the headspace you were in. They are often like, 'Fine, voice-match it, totally fine with that.'"

But some big stars are happy to do it. Fox recalls Meryl Streep coming in to do her ADR for the film "August: Osage County," and Matt Damon and Ben Affleck showed up to do it for "Jay and Silent Bob Strike Back." Fox remembers asking Damon why he was doing it, and the actor's answer was simple: "I'll do anything for Kevin Smith," referring to the film's director.

But some stars can't stand the process. Fox handled ADR for the indie "Rolling Kansas," starring Thomas Haden Church. The actor was so baffled by having to voice clean audio that he decided to just say whatever was in his head.

"If the original line was 'bulls--t,' he would replace it with 'peanut butter and jelly,'" Fox said. "It was funny. The TV version was just ridiculous."

2 voice actors who pretend to be Cameron Diaz and Owen Wilson

Married couple Jessica Gee-George and Grant George are veteran voice-over actors who have imitated stars including Cate Blanchett and Owen Wilson over the years.

They say the process usually begins with an email from a postproduction supervisor about the actor they would match and how many lines they would perform. Sometimes they simply agree to the job if the supervisor knows they can do it. Otherwise they audition for the voice match, sending out a file they recorded at home.

If they get it, the job is usually no longer than a four-hour day. They record in a studio with the footage in front of them on a big screen. The director often walks them through the lines, ranging from screaming for hours to saying a few lines that got garbled. The work is often needed for action scenes.

"Whether it's a fighting scene or it's a close-up of someone breathing, the actors don't come back for that type of work," Gee-George said, noting that she did Cameron Diaz's screams and gasps in the car-crash scene in 2001's "Vanilla Sky."

The recent trend, however, is voice-matching in trailers for big films that kick off publicity over a year before release. Studios will rush to get out small teasers without any audio ready for the footage they want to use. That's when the voice-over artists get a call.

"They often need a nice, clean line of dialogue," George said. "Sometimes they even use our voices as a temp for when they send the trailers for approval to the studio."

When a dead actor needs to be brought back to life

Voice matching also comes up when an actor is deceased. Stephen Stanton is a go-to guy when a film, TV show, or video game needs the voice of an actor who is no longer with us.

He's currently the voice of Peter Cushing as Grand Moff Tarkin in "Star Wars Rebels" and Alec Guinness as Obi-Wan Kenobi in numerous "Star Wars" games. He was also the voice of film critic Roger Ebert in the documentary "Life Itself" and legendary horse-racing announcer Chic Anderson in the movie "Secretariat."

"This is not about being an impressionist," Stanton told Business Insider of voice matching. "You're all of a sudden being put into the position of being the lead actor in that film for a day, so you've got to get into the actor's head and into the script."

You also have to be able to match up perfectly with the lips on-screen, which goes a lot further than just getting the vocal tone right. And Stanton, who boasts being able to do more than 200 voices on the spot (here are some of them), says you have to be ready to work at a moment's notice.

"Sometimes a trailer house needs you in 10 minutes. They are in a real crunch," he said. "They are putting something together and it has to get to the studio for approval. There's no real rehearsal with something like that — you can either do the voice or you can't."

The legendary actor who didn't do his own voice on a 'Lion King' song

There's no better example of 11th-hour voice matching than Jim Cummings' work on "The Lion King."

The voiceover actor remembers hanging around the studio one day when he was summoned by Tim Rice and Elton John. They were mixing a song for the Disney classic, specifically "Be Prepared," performed by the film's villain Scar, voiced by Jeremy Irons.

"They wanted me to take a crack at doing the song," Cummings told Business Insider. "As one of them put it to me, Irons singing sounded like, 'You could hear every Marlboro the man has ever smoked in his life.'"

Cummings had been the lead singer for The California Raisins, the animated musical group, during its heyday in the late 1980s. And he has been the voice of Disney's Winnie the Pooh and Tigger since 1987, among many others.

Though he had never done Irons' voice before, with only eight days before the premiere of "The Lion King" in theaters, Rice and John needed someone with a better singing voice but who could still sound like Irons.

"The way I saw it, if I stink, we're going to know pretty quick," Cummings said. "So I did it, and apparently I nailed it."

But there was still one last hurdle to cross. Jeffrey Katzenberg, then the head of Disney's animation studio, had to sign off on the performance.

"They went in and played the song," Cummings said. "And Jeffrey was like, 'That sounds great, it's fantastic. I thought you guys were worried Jeremy [Irons] wasn't going to be able to pull it off.' And they were like, 'So you like it, Jeffrey?' Like four times they asked him, and he's like, 'It's great,' and then they said, 'Great. By the way, it's not Jeremy.' And Jeffery was like, 'What?!' And they explained what had to be done and me coming in, and he was like, 'I still like it.'"

Cummings has also filled in on songs for Christopher Lloyd in "Anastasia," Russell Means in "Pocahontas," and Danny DeVito in "Hercules."

"It's a tough thing when you're in the studio with a guy who's really talented and he's a great actor, but he just can't sing," Cummings said. "It can be frustrating for everyone. But voice actors are character actors, and that's just what they sound like. It's the character first and the voice second, because you have to be true to the character and flesh him out and make him real."

Working in the shadows

"It's a profession that's right in front of your face, but you don't see it," Cummings said of voice-matching work.

The actors are rarely credited for their work, and if they are, it's usually with an "additional voice" credit.

"It's a facet of the illusion of movie and TV making," Grant said. "It's like when people are shocked a scene was shot on a sound stage instead of in a house."

Though voice-matching gigs are fairly regular, it's hard to make a living on them. They pay from a union scale of about $900 to somewhere in the four-figure range for a day's work. That's regardless of whether the voice is used; if it is used, the voice actor also gets residuals on the project.

Often voice-matching artists won't know whether they can be heard in the movie until they see it themselves and try to catch where their performance is.

Though it's a job these performers will hardly ever get any recognition for, all of the voice actors interviewed for this story said they were doing their dream job and loved the fact that few people knew what was going on behind the curtain.

"I don't think audiences know this is going on at all, and that's the whole point of it," Stanton said of voice matching. "If everyone is doing their job well, this is done very seamlessly, and no one is aware that it's happening, and that's all part of movie magic. You don't want to take people out of it."