In films, the star is often replaced by a double in many situations, either because the task involved is too dangerous to risk injury or the star simply cannot or will not do what the role requires. We in the audience almost never notice the switching back and forth. One might think that this is solely due to the care that the film makers take to make sure the double has the same build as the star and is made up to have the same exterior appearance in terms of clothes and hair, and by avoiding close ups of the face.

But it turns out that there is more at work and that the brain of the viewer helps to maintain the illusion. In a recently published article, researchers “tested whether the visual system facilitates stable perception by biasing current perception of a face, a complex and behaviorally relevant object, toward recently seen faces” and “found that perception of face identity is systematically biased toward identities seen up to several seconds prior, even across changes in viewpoint.” The article is behind a paywall but you can read the abstract here.

This article explains more about the research.

Researchers have pinpointed the brain mechanism by which we latch on to a particular face even when it changes. While it may seem as though our brain is tricking us into morphing, say, an actor with his stunt double, this “perceptual pull” is actually a survival mechanism, giving us a sense of stability, familiarity and continuity in what would otherwise be a visually chaotic world, researchers point out.

“If we didn’t have this bias of seeing a face as the same from one moment to the next, our perception of people would be very confusing. For example, a friend or relative would look like a completely different person with each turn of the head or change in light and shade,” said Alina Liberman, a doctoral student in neuroscience at UC Berkeley and lead author of the study published Thursday, Oct. 2 in the online edition of the journal, Current Biology.
…

“Our visual system loses sensitivity to stunt doubles in movies, but that’s a small price to pay for perceiving our spouse’s identity as stable,” said David Whitney, associate professor of psychology at UC Berkeley and senior author of the study.
…

Without the extraordinary ability to recognize faces, many social functions would be lost. Imagine picking up your child at school and not being able to recognize which kid is yours,” Whitney said. “Fortunately, this type of face blindness is rare. What is common, however, are changes in viewpoint, noise, blur, and lighting changes that could cause faces to appear very different from moment to moment. Our results suggest that the visual system is biased against such wavering perception in favor of continuity.”

The idea that our brain tries to smooth over discontinuities also helps explain why we are not bothered by jump cuts in films where the scene instantaneously switches from one to another, which, if one thinks about it, one would think should be quite jarring.

In his classic book on film editing, In the Blink of an Eye, Walter Murch writes about the violence of the cut. In an instant, everything you see onscreen is erased and replaced with something else. Often the scene jumps to another place or time. “Nothing in our day-to-day experience seems to prepare us for such a thing,” Murch writes. And yet, in movies this happens all the time, and we accept it without giving it a second thought.

It’s amazing that film editing works, because it’s so disruptive to the visual information coming into the brain, says Jeffrey Zacks, a neuroscientist at Washington University in St. Louis. On the other hand, Zacks says, our brains do quite a bit of editing of their own—and we’re every bit as oblivious to that as we are to the film editor’s cuts.

For example, our eyes are constantly darting around from place to place. We do this thousands of times a day, and every single time, during the fraction of a second that the eyes are in motion, our visual system essentially shuts off to spare us the nausea-inducing blur. The movie playing in your mind’s eye may feel smooth and continuous, but that’s an illusion manufactured by your brain. It cuts your visual stream of consciousness right before your eyes jump and joins it to what comes right after.
…

In another experiment, Zacks and his graduate student Khena Swallow found that people tend to struggle to remember objects they’ve seen just before an event boundary. Zacks demonstrated this at the Academy event with a scene from the French film Mon Oncle. Not much happens, really. A man walks through a door into a house. But that action represents an event boundary, and when Zacks asked the audience to call out which of two objects—a cat or a chair—was present in the scene before he walked through the door, only about a third of the people correctly picked the chair (another third picked the cat, while another third stayed quiet, presumably because they had no clue). Even though the chair had been plainly visible just a few seconds earlier, most people failed to recognize it.

This desire of our brain to preserve continuity leads us to often overlook actual mistakes in continuity in films, such as Harry Potter’s T-shirt abruptly changing in the “Order of the Phoenix”, unless the mistake is really obvious. After I watch a film, I sometimes go to the site IMDb website where for each film, there is a section where all the goofs are listed, including those of continuity. I find that I have almost never have noticed them myself, which is a good thing, since being noticing them would likely have marred my enjoyment of the film.