Tom Cruise's former love interest in "Mission: Impossible – Dead Reckoning Part One" almost looked totally different.

The movie features a brief flashback to 1989 for a scene that involves Cruise and his character's girlfriend. As it is now, the scene plays out without showing Cruise's face and has Mariela Garriga playing his girlfriend, Marie. But director Christopher McQuarrie originally had a different idea to cast Julia Roberts as the girlfriend and de-age both her and Cruise.

The reason this didn't happen was the price tag.

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"I said, 'OK, if I were doing this sequence, it would be Tom in, say, 1989,'" McQuarrie explained on the "Spoiler Special" film podcast.

A split photo of Tom Cruise and Julia Roberts

Tom Cruise nearly appeared with Julia Roberts, both de-aged by 30 years, in the latest "Mission: Impossible" movie. (Bryan Bedder/Getty Images for Paramount Pictures/Steve Granitz/FilmMagic)

The first "Mission: Impossible" was released in 1996, and he theorized if it were to come out in 1989, "Top Gun" director Tony Scott would have been a good pick to direct it.

"We looked at ‘Days of Thunder,'" he said, referring to another of Scott's films, "and we looked at the style of it, and we started thinking what would it look like if Tony Scott had shot this, and who would it have been?

side by side photos of Tom Cruise and Julia Roberts in 1989

Tom Cruise and Julia Roberts in 1989.  (Ron Galella, Ltd./Ron Galella Collection/Steve Granitz Archive/WireImage)

"I looked back at who was the ingénue, who was the breakout star in 1989? And right around then was ‘Mystic Pizza.’ And I was like, ‘Oh my God. Julia Roberts, a then-pre-’Pretty Woman' Julia Roberts, as this young woman.'"

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black and white photo of Julia Roberts from 1989

Superstar Julia Roberts poses for a portrait in 1989 in Los Angeles. The "Mission: Impossible" director almost asked Roberts to make a cameo in the latest film. (Aaron Rapoport/Corbis/Getty Images)

While he seemed excited about the idea of getting Roberts for this cameo, he did say, "The only way I could have seen doing the sequence justice was to somehow convince Julia Roberts to come in and be this small role at the beginning of this story."

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The technology used to make actors realistically appear so much younger — a good recent example that did make it to the final cut is Harrison Ford's flashback scene in "Indiana Jones and the Dial of Destiny" — is not cheap. As McQuarrie noted, it also becomes a major talking point for the film.

A still frame of Harrison Ford as Indiana Jones in a dirty blue jacket looking up and out ominously, his face is altered by AI

Through the use of AI, Harrison Ford appears in the new "Indiana Jones" film as if he is 35 years old. (Disney/Lucasfilm)

"As you're conceptually going through it, you're like, 'Now all anybody's going to be doing is thinking about the de-aging of Julia Roberts, and Esai [Morales], and Tom and Henry Czerny,'" he said, referring to the other actors who would have been in the scene.

Tom Cruise smiling at movie premiere

Tom Cruise, 61, still does his own stunts in the "Mission: Impossible" films. (Don Arnold/WireImage)

"And then I got the bill for de-aging those people before their salaries were even factored into it," he added. "And if you put two of them in a shot together, or three of them in a shot together, it would have been as expensive as the train by the time we were done."

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The train stunt McQuarrie mentioned is in the climax of the movie. According to Variety, crew members built the train carts from scratch and then created an actual crash.