Marquee Alter Ego

If a character is reasonably popular, through cartoons, comics or general folklore, they'll probably have some kind of iconic visage. In the world of Hollywood, however, any actor with the clout to play the character in question is probably recognizable to the degree of bankability. Or the actors just want to the audience to see their face. (Or have difficulty emoting without it.) Or the heavy makeup or costuming is just too uncomfortable. Sometimes they just feel a character in crazy get-up just looks too silly doing anything but fighting.

As a result, our strange-looking character is reverted back to a less costumed face (often by way of an Anti Climactic Unmasking), and may stay that way until the film's climax. Keep in mind this isn't just about character.

It's not so strange when you consider how important an actor's face is in "selling" the acting in media such as film where body language matters. Thus, it is not surprising that directors might want to be sure the audience has an unobstructed view of that important face at any especially dramatic moment. Comic books or prose narratives offer other methods of conveying the same information, rendering it less important to see the character's true face.

Compare Adaptational Attractiveness.

Film

 * Both sequels to The Santa Clause feature a plotline that turns fat, old, jolly Saint Nick back into Tim Allen.
 * While the back-and-forth transformations between Ben Grimm and The Thing are a staple of early Fantastic Four comics, the movies spend a lot of time with a non-deformed Dr. Doom. The second one goes to especially contrived lengths to get Julian McMahon out of his metal mask.
 * Mystique from X-Men is played by world famous supermodel Rebecca Romijn, and has the power to take any form. Do the math. Commendably, though, she spends surprisingly little time as herself. (But that leg-shot in X2 is enough...)
 * However, in X-Men: The Last Stand, we're only teased with the Beast being turned human. They might just not have wanted a very famous sitcom star showing up in a dark superhero movie.
 * And in X-Men: First Class, both Mystique (now played by Jennifer Lawrence) and Beast (Nicholas Hoult) spend more time in human guise than in their blue skinned/furred forms.
 * Not transformed back, but after Jack Nicholson becomes the Joker in the 1989 Batman film, there are stretches where he puts on enough makeup to pass for human.
 * Also minorly subverted, as the fact that he still has his permanent Slasher Smile is enough to drop him in the Uncanny Valley.
 * Used with Batman himself, too. In Batman Returns, he takes his mask off when trying to talk Catwoman down from killing Schreck.
 * While he is not particularly abnormal looking outside the costume, the sequel to Spider-Man has a plot element where Peter loses his powers, allowing a good chunk of the movie to have Peter just be Peter.
 * Also happens in the end of each movie. Excuses are constantly made to show his face. Usually thanks to rips and tears that are magically repaired by the credits.
 * In the third movie, we get a token few minutes of Venom's wonderfully creepy tooth-filled maw before he starts peeling back his "mask" every time he speaks - probably to give Topher Grace some more face time.
 * In the Lord of the Rings movie trilogy, Andy Serkis felt depressed about his groundbreaking work as Gollum being relatively anonymous. So the filmmakers shot a flashback scene as Smeagol for him.
 * This is however the only time we see Serkis as normal.
 * Strangely, the sequel to Shrek has Fiona turning back to normal for a while--maybe just to get mileage out of the model. Maybe the animators hoped to use it in promotional material that wouldn't spoil viewers who hadn't seen the first one yet.
 * Sylvester Stallone only spends about ten minutes with his helmet on in Judge Dredd, despite the fact that in the comics we have never seen Dredd with his helmet off.
 * Likewise, Billy Zane gets a lot of face time in The Phantom, considering he's playing a character whose face is never shown clearly in the comics.
 * Iron Man: While Tony is seen quite a bit out of the suit, having his mask come off in the final fight seems to be for this reason.
 * And many shots show Stark's face from inside the Iron Man helmet, although that may just be because shining some lights in Robert Downey Jr's eyes is much cheaper than fully animating the suit in flight.
 * Does at least serve to make the in-flight conversations with Jarvis slightly less Talking to Themself-ish.
 * Such "In-Helmet" shots are also relatively common in the source material, however. Tony Stark is a Marquee Alter Ego in the comics, too?
 * Likewise, in Captain America the First Avenger Steve Rogers spends a lot of time not wearing the half-mask hood—sometimes as himself, and other times with it simply pushed back. At one point he wears a helmet instead.
 * The Red Skull spends about half the movie wearing Hugo Weaving's face before he finally dramatically peels it off.
 * The Avengers: Many of the heroes spend a great deal of time unmasked; Tony Stark and Captain America as above. Bruce Banner spends most of his time as a human also, though that probably doesn't count (and given that Ruffalo did motion capture for his Hulk form, this might be considered a partial aversion).
 * Daredevil has his mask ripped off right before his final fight with Kingpin. At least Ben Affleck was willing one to wear one at all, unlike Colin Farrell's Bullseye.
 * Inverted somewhat in the film adaptation of Watchmen, where Rorschach spends more time in costume than he did in the comic, specifically the prison break scene (where he originally was unmasked)
 * During one scene of Pirates of the Caribbean: At World's End, we see Bill Nighy instead of Davy Jones.
 * Averted in V for Vendetta where Hugo Weaving's face is never seen, except at the end just before the credits when the crowd takes off their masks. By that point,, so that's not even the character V - just Hugo Weaving as some bloke.
 * Also, you're actually seeing James Purefoy some of the time; he was replaced by Weaving after filming a few scenes, which they didn't bother to redo because of the mask.
 * In The Fugitive the plot calls for Dr Kimball to spend most of the movie in disguise; the filmmakers deliberately had him start out some distance from Harrison Ford's usual look, so that when he disguised himself by dying his hair and shaving off his beard he became the Harrison Ford audiences were paying to see.
 * In Comic Book: The Movie, the hero appeals to Bruce Campbell, starring in a movie about his childhood idol superhero, to make the movie about the original character rather than the recent Darker and Edgier version. He mainly appeals to Campbell's ego, saying the classic costume would allow far more of Campbell's face to be seen than the new one.
 * Universal Pictures executives wanted this trope to apply to Jim Carrey when he played The Grinch, pushing for less-Seussian makeup than what Rick Baker had designed, and Carrey and director Ron Howard got so fed up with this that one makeup test they sent for the executives to consider was just Carrey painted green. The executives finally accepted the elaborate Grinch makeup/costume, the movie was a huge hit and Baker won an Oscar.
 * In Dark Passage, the entire first act is done from the POV of the main character and we're not shown his face. Then he has plastic surgery, and when the bandages come off we see it's now Humphrey Bogart.
 * In the video game Splinter Cell: Conviction, Sam Fisher (who is on the run after the events of Double Agent) starts the game wearing a dressed-down ensemble and his standard gun, which is a far cry from the iconic infiltration suit getup her wore in the first three games. Over the course of Conviction, though, you'll eventually revert back to your original getup by obtaining several key pieces of equipment (including your signature gun and trademark goggles).
 * In the Harry Potter novels, Death Eaters wear masks when attacking, though the main characters can sometimes guess who's behind the mask by their voice. In the movieverse, the Death Eaters are played by good-looking actors with vocal fanbases so of course they are always whipping off their masks, even though the whole point of disguising themselves is so they can get away with their criminal behaviour.
 * In the rather strange and critically-panned film adaptation of The Spirit, we see plenty of Samuel Jackson's face in his role as The Octopus. In the comics, The Octopus' face was never, ever shown.
 * Justified in Green Lantern, where Hal Jordan's domino mask explicitly only appears when he's trying to hide his identity. (Hilariously, in one scene where he does try and hide it from his love interest, it fails spectacularly as she quickly sees through the act.)

Live-Action TV

 * Seen often in Smallville. Green Arrow seems to spend more time out of his hood and sunglasses than in them while in costume . And this could be the reason why the writers took so long to give Clark in his mild-mannered, glasses-sporting civilian disguise.