In Captain America: Civil War, one scene shows a young Tony Stark in a pivotal moment with his father, which eventually plays into the movie's story.
But exactly how did the special effects team of Civil War create that?
They watched movies from the 1980s, particularly several that starred a younger Robert Downey Jr.
The movies they paid the most attention to were Weird Science, Less Than Zero and Back To School, which all featured the actor.
"We went a little closer to [his look in] Less Than Zero," visual effects supervisor Dan DeLeeuw said to The Wrap. "No one wanted the spikey hair from Weird Science like I did!"
Although the scene in Civil War only lasts 3 minutes on screen, it consists of a whopping 4,000 frames. Even more challenging is that the older Stark appears with the hologram of his younger self, together on film at the same time.
For that scene, the production used two doubles: one for adult Stark and one for the young Stark. Downey, though, played both parts, opposite each double, with his younger version requiring motion capture equipment attached to his face. He had to mimic the performances of the stand-ins for both doubles, too, which at one point, also meant shaving off Stark's mustache and goatee to stand in for his younger self.
Here's a photo of Downey in Less Than Zero:
Fortunately, Downey has a long history of movies and was in the business when he was about the age the younger Stark is in the hologram that viewers see in Civil War. So mostly using footage from him in Less Than Zero, the effects artists did several passes of his younger face over his older face until they got it just right.
"Each pass is sculpting in a way," said DeLeeuw. "Pulling off material, pulling off age ... When it feels right, you go back and clean it up."
The result looks pretty impressive in the final product, Captain America: Civil War, which is playing in theaters now.