A character's mask is usually intended to obscure the face. In animation this used to also be a helpful cheat to draw something similar.
In order to look less wooden it is very common for the mask to become a character's face. The eyeholes and even the shape of the mask will contort seamlessly depending on the character's mood. At the very least the mask will move as if painted on. This is especially true when a character's eyes are not drawn while in costume.
This can even extend to eyeglasses (especially Eye Glasses) and similar accessories. The lenses of an expressive character become organic extensions of the body: widening with surprise, narrowing with suspicion, and scaring with shiny. Interestingly cracked glasses can autorepair thanks to this property between scenes. This is similar to Open The Iris, where the iris, not the pupil, widens and shrinks.
Even if the mask, helmet, or head itself isn't very expressive, you can still get a lot of mileage out of Disembodied Eyebrows. For other emotion-conveying clothing see Expressive Shirt.
A peculiar variant of this extends to characters who only have a skull for a head, and therefore have Expressive Eyesockets.
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Commercials for the Jack-in-a-Box restaurant. The mascot's head is a giant ping-pong with a pointed nose and painted-on face, which always wears an expression suited to his tone of voice, though the actual transition only occurs when he's off-camera.
Anime and Manga
A great example is Kinnikuman, where the main character and just about everybody on his Planet of Hats wears a skintight mask meant to completely replace his face. It follows each and every one of his expressions perfectly. Due to the series being primarily about Professional Wrestling, several other characters wear masks of varying properties (Mystic wood, crushed sapphire/steel alloy, etc.) that are likewise expressive, just not to the degree that Kinnikuman's is.
Alphonse Elric from Full Metal Alchemist could be remarkably expressive with his helmet, despite the fact that it never moved (even when he talked; his voice came from inside the armor). He's actually able to blush visibly, despite being a soul bound to an empty suit of armor. The helmet only changes when he's drawn as chibi. Which happens fairly often — possibly because of this trope. The rest of the time it never changes.
The villain Kain in the first Tenchi Muyo! movie had a face that resembled a mask (including not moving when he spoke) and which changed expression occasionally, though it seems to have been a part of his body.
Shinigami-sama from Soul Eater has a mask in which the eyes change shape from round and dopey to triangular when he is about to beat an enemy into the ground. And for a masked character who doesn't seem to have a body inside his costume, he also manages cross-popping veins and tears.
Naruto: Kakashi is a notable aversion of this: the entire bottom half of his face (nose to chin) is always covered by a mask and his right eye is covered by his ninja headband most of the time, neither of which moves more than they realistically would. You can usually very easily tell his mood from just that one eye and its eyebrow. The immobile mask thing has improved slightly with Shippuuden, so now we generally see some movement when he speaks.
Tobi's almost completely face-concealing mask is likewise immobile, and he mostly expresses himself with body-language or, after he reveals himself as Madara, a close-up of his eye.
In Saiunkoku Monogatari, Kou Kijin's masks themselves aren't animated, but he has a wide assortment of them made by his friend and colleague Kou Reishin, and some of them are very expressive.
Averted by Usopp's Sogeking mask in One Piece, which generally hides his expressions to make him look calmer, but even then you can see his jaw sticking out from the bottom of it in a Wild Take.
Brook can do this with his skull head. This happens particularly often whe he's in battle, when he'll often have angry eyesockets.
Arkana's mask in Yu-Gi-Oh! fluxes with his eyes. Since the mask is wider than his head, when his eyes are open large, it can look like Arkana's eyes are wider than his head.
Some Hollows in Bleach manage to be rather expressive despite the fact their face is essentially a bone mask. This is helped by the changes to the glowing pits of fear that are their eyes, narrowing and widening to help convey their emotions.
In Skip Beat!, Kyoko's Bo costume (a giant chicken suit) is somehow capable of blinking, glaring evilly, and a wide variety of other expressions.
No Face's noh mask in Spirited Away was meant to be a complete blank and rely on tricks of lighting to convey mood, both as real masks do and to suggest his lack of individual personality. It wound up betraying some emotion, though, with the mouth and eyes seeming to tilt up and down slightly.
Batman's mask can sometimes be seen doing this. For Bats it's almost always "brow furrowed." Almost◊. The appearance of this is explained by the live-action films: close-ups on Batman's face make clear that he's wearing eye makeup in the gap between his mask and the edges of his eyelids. (This is the most likely method for any live actors masked in the same style.)
Another good example is Cassandra/Batgirl II's mask: Unlike Batman's it's full face and even the eyes are black, but they get a lot of mileage out of changing the shading on it.
Spoilers mask is similar to Spider-man and Deadpool below◊, complete with the cocked eyebrow◊
Destro in G.I. Joe was a very good example of this, given that despite his wearing of a metal mask that completely covers his head, it was in essence "painted on", in regards to being able to furrow his brow, smile, frown, move his lips, etc. Destro's mask was played with in the live-action film where his severely burned face is turned to flexible metal through the Applied Phlebotinum of the nanomites.
G.I. Joe: Renegades: Destro is a subtler example, as only the parts surrounding his eyes move and even then it's so little that it's only ever shown in close-ups.
Spider-Man and Deadpool, both of the Marvel Universe, often manage some pretty goofy expressions even though they wear masks concealing their entire face. These are usually of the "cocked eyebrow" variety. Depending on the Artist, Deadpool's mouth can be seen through his mask, making his expressions even goofier.
Most dramatic is when Spider-Man narrows his eyes. Somehow this makes the whites of his mask narrow as well. (During the Mc Farlane big-eyes era, this was particularly emphatic.)
They experimented with this for the movie, but decided that in live-action it moved the mask firmly into the Uncanny Valley.
Both Marvel Universe characters Doctor Doom and Iron Man have masks specifically described as made of metal, yet both can show emotion when needed. The trick is the angle from which they're shown.
Justifications exist: Iron Man is shown creating a faceplate for his armour that follows his own expressions, the better to intimidate his foes. In The Ultimate Super-Villains anthology Doctor Doom's updated mask contains micro-servos that can mimic expression. He habitually keeps it locked in an arrogant scowl to better intimidate his underlings.
The Spirit has a domino mask that's very flexible and shows expression very well.
V for Vendetta: His mask does not move, but lighting effects are used to make it more expressive. If you don't show the eyebrows, he looks innocently happy, but focusing on them makes him look more formidable.
In Fall Of Cthulhu by Boom! comics, the Masked Mute IS this trope. Her communication is literally all the different expressions her masks make.
The Taskmaster wears a skull mask that is quite expressive, to the point where his mouth seems to move when he talks. Depending on the Artist is will either contort with his expressions like a rubber mask, or rely on the expressions of his eyes if it's a more hard mask.
Rorschach from Watchmen, whose Inkblot mask changes depending on his emotion at the time.
There's also Emoticon from Welcome to Tranquility, who is something of a unique example. His mask actually always shows his true emoticons, and is effectively a proxy face for him, given a supervillain by the moniker of The Typist actually mutilated his face beyond recognition and rendered him utterly blind. The mask also has prosthetic eyeballs in it, allowing poor Emoticon to see. Understandably, he's got a bit of an attitude problem. Poor guy.
In general, more comedic fan art featuring the Pyro from Team Fortress 2 will make its gasmask expressive, since it is otherwise The Faceless.
The masks in The Incredibles are essentially dark patches on the characters' skin, changing shape as their features move and even covering their eyelids. Given everything else their costume designer, Edna Mode, is able to make their costumes do, this might just be another feature.
All characters in the BIONICLE films (save for a few such as Krekka who do not wear masks).
Aversion: The character designers for the animated film The Iron Giant deliberately gave the title character's head extra hinges, shutters, etc. so that it could produce facial expressions without cartoony contortions. The live-action Transformers movies are similar, with even greater detail.
While for the majority of V for Vendetta movie V's mask is just a mask, several scenes have been digitally edited so that the mask moves very, very slightly. The idea was, because people are so used to masks not moving, if they saw the eyebrow go up an unnoticeable fraction, it would make a huge subconscious difference. And it did, obviously. Watch the mask just before the big fight towards the end. It helps that it already looks a bit CGI in its normal form, but watching closely you can see it move the tiniest bit.
Justified in The Mask (all versions) because the thing is magic, even if it appears wooden when not being worn.
Played for laughs in Scary Movie, where the expression of the mask worn by the killer changes between shots into anything from mad, to happy, to stoned.
Jack Skellington in The Nightmare Before Christmas not only has expressive eyesockets, he even can stretch his face into aterrifying roar◊. This helps to make him more endearing to the audience, obviously, but some fans have come up with an in-universe explanation that, this paired up with the fact that Jack apparently has a need to eat and can feel pain, that Jack is a skeletal creature (Bone Demon) rather than an actual skeleton.
In The Belgariad and its prequels, the god Torak wears a steel mask over a severe burn that shifts as his healthy face would. To his defense, he is a god.
Roald Dahl's book The Witches gives the Grand High Witch of All the World a mask that looks exactly like a beautiful human face, moving perfectly with her own, which is ugly and scabby. The improbability of such a mask is commented upon.
Discworld's Death explicitly doesn't do this in the text itself. His skull is always described as "grinning". However, Paul Kidby's illustrations of the character can be pretty expressive. They still grin, but subtle changes around the "eyebrows" (or at least the place where eyebrows would be if he had them) play a big part.
The character of fallen and tragic Prince Gaynor the Damned turns up in several of Michael Moorcock's novels that feature an incarnation of the Eternal Champion. Prince Corum opposes Gaynor, and so does Elric at one point (in The Revenge of the Rose). Prince Gaynor the Damned is described as being sheathed from head to toe in metal armor which changes colors constantly, so that nothing of his body can be seen, and carries a great shield with the eight-arrowed star, the sign of Chaos. The colors that play over the surface of Gaynor's armor appear to change in reaction to Gaynor's emotions and moods. Indeed, Gaynor was cursed with eternal torment for betraying the cosmic balance and is unable to take off the armor on his own (although in one of the books, Corum rips off Gaynor's helmet and reveals a face continuously transforming in accordance with his status as a being of Chaos; his flesh seems to writhe like a mass of maggots). Gaynor is immortal (and may be unkillable, too, as he alone survives a wound from Elric's soul-sucking runesword Stormbringer), but longs only for death.
Live Action TV
In the Tales from the Crypt episode "Only Skin Deep", an abusive creep goes home from a costume party with a shapely young lady - himself dressed as a pirate, her as a body bag ('artificial shell with a corpse inside' - tip off number one?) and with a slightly disturbing mask. Naturally, he doesn't notice that the mouth of the 'mask' and the eyeholes move when her mouth and eyes do, respectively... typical Crypt type 90's horror ensues. It's clearly makeup, but for plot purposes, no one has ever apparently put on corpsepaint in their life, god forbid for a costume party...
The puppet's sunglasses in the original Mahna Mahna video change shape to match his expression.
Calvin And Hobbes - Spaceman Spiff, interplanetary explorer extraordinaire, wears a space visor that is more like a small square black screen perched on his nose, which somehow conveys all of his eye's expressions. The standard expression is two squares, but he uses circles for alarm, a thin line for when he's squinting, and triangular shapes for when he's angrily blasting stuff. For that matter, the Calvinball masks and Stupendous Man's mask are very expressive for pieces of cloth (the Stupendous Man hood, however, does white out Calvin's eyes in his daydreams.)
Exalted has a somewhat terrifying example in the Mask of Winters. He wears a mask that covers his entire head — one side has a diabolical grin, while the other side has an expression of sorrow and fury. You can tell what mood he's in by which side is facing you — and he'll always be facing you, as he can reverse every joint in his body at will.
In Dungeons & Dragons there's the Hat of Disguise, which can, when activated, become any type of headgear; thus, it could become a mask, the expression on which could be changed each round.
The Swiss theater troupe Mummenschanz play with, deconstruct, mash back together, and otherwise have all sorts of fun with this trope.
The masks used in ancient Greek theater had all sorts of exaggerated expressions, but averted this trope: they were fixed facial expressions that cannot change. The actor used his or her body to convey emotion, and nothing else.
Other theatre traditions, including the Italian Commedia del Arte, also avert the trope with Greek theatre-like masks.
The masks of Japanese noh theater are made to change expression based on how you tilt your head.
An Expressive Mask can be created simply by drawing the mask onto the performer's face with makeup. As well as being cheap and easy, this method doesn't hide the face like an actual mask, which could otherwise inhibit the emotion the performer conveys.
Psychonauts: Not quite a mask, but Agent Sasha Nein's glasses are capable of changing shape with his expressions. He ispsychic, of course, but that seems a rather trivial use of telekinesis.
The Shy Guys from the Super Mario games do this, at least in Mario Strikers Charged and Paper Mario.
Bandits also fit this trope to a T. The mask esssentially is their face.
The human characters in Viva Piñata wear tribal-style full face masks that move and emote perfectly. However, given the cartoonish style of the game and related tie-ins, it's possible that these "masks" are their actual faces.
Legion in Mass Effect 2 has several articulated plates around his single eye, capable of approximating organic facial expressions like surprise or interest. The effect is similar to Iron Giant and Transformers mentioned above.
The mask Sly Cooper and the gang wear are quite expressive, Sly is never seen without it!
Monkey Island: Murray the evil demonic skull]] has expressive eye sockets.
But the same strip contains robots whose normally-oval eyes appear as semicircles when they're annoyed, or as lines. (The latter made sense when it appeared on Sawtooth Rivergrinder, however, as the ^ shapes were displayed inside his eyes, which are confirmed to be capable of functioning as a graphical display.)
Xykon of The Order of the Stick doesn't wear a mask, but his skull is surprisingly expressive, even if it can't do an Evil Laugh. Of course, the rest of his body is also moving more than a normal skeleton.
"Suck it, arthritis!"
Averted in The Adventures of Dr. McNinja The masks on the titular character and his ninja family stay motionless. However, they manage to be fairly expressive through eye movements (it helps that the masks show eyebrows.)
Any inconsistencies with the masks have gotten a preemptive Hand Wave: The Doctor and his family know a variety of "ninja tricksh" which, among other things, let them eat with their masks on.
This effect shows up on the Johnny Wander characters who wear completely opaque glasses; it's subtler than most, but the glasses narrow and widen with emotion and sometimes change shape. Except George's, which are always perfectly round. Yuko demonstrates!
Strong Bad from Homestar Runner, in his first ever e-mail. In response, he claimed that his mask and gloves were his face and hands. 161 e-mails later, he was "shown" removing his mask offscreen with a ripping noise, and making painful gurgling sounds until he puts it back on, after which he swears "I am never doing that again!".
Parodied in Red vs. Blue, which can't do this because it's Machinima in Halo, where everyone wears a helmet. Even talking is merely head-bobbing. When the Reds see the Blues talking on the screens under the canyon, but can't hear them, they comment on how ridiculous people look because you can't see their emotion. While vigorously bobbing his head.
In fact, Machinima in general is all about getting by without live actors or this trope. Voice actors and careful posing of characters have to sell the emotion instead.
Not in canon, but fans love to take the Pyro's gas-mask and deform it according to this trope.
Steel's mask shows his expression in the comics as well. A lettercol suggested it was either evidence of his superpowers (a Retcon that was later dropped) or he had really strong facial muscles.
Spider-Man shows in particular traditionally seem to love using the mask to not only show but also highly emphasize the character's emotions and expressions. Spider Man the animated series was fond of using the mask for expressions of surprise, including one instance where the eye-parts of Spidey's mask become so large that they take up most of his face.
This extends to the The Spectacular Spider-ManAnimated Series, which also takes a lot of pleasure in the marvelously emotive full head masks of the Green Goblin and the Chameleon. Chameleon at least has a good excuse, but we've also seen Flash with a donkey head mask during a play, the mouth of which moved just as fluidly.
Also noteworthy from the animated continutity are Atomic Skull and Blight; both have exposed skulls (the latter because of transparant skin, the former because...well...) which tend to show a greater range of emotion than you should be capable of getting from a skeletal jaw and eye-sockets.
Music Meister wore a visor-ish mask that was detailed to look like a measure of music with a pair of one-eighth notes on it. It was quite expressive, with the notes functioning as pupils and the bar connecting them flexing like a mono-eyebrow. And, of the over a dozen costume changes he does in the course of one episode, is the one part that NEVER CHANGES.
The coffee-table book Batman Animated reproduces the official model sheet for Harley Quinn, which explicitly instructs that the top of the mask is intended to change shape with her facial expressions ("Think of it like eyebrows").
One aversion of this trope in Batman The Animated Series is Page Monroe's completely unmoving mask in "Mean Seasons," which parallels her aversion to her actual face.
Lampshaded in one episode where AndrAIa assumes Hex's identity. She sits in front of a mirror waving her hand back and forth in front of her face, watching the expression change and saying "Happy...sad! Happy...sad!"
> "INTRIGUED! I've never been more in touch with my emotions!"
In an episode where Bob actually removes Hex's mask, we see that there is nothing underneath. The mask is her face, and her expression is at the mercy of chance. This is why, in the third and fourth seasons, after Hex becomes sane and does a Heel Face Turn, her face does become expressive and actually moves when she talks.
The goggles that Time Squad officers wear in Time Squad act in this way. The easiest example would be Buck Tuddrussel.
Most Transformers faces were like this. Despite being made of metal, they would easily slide into various facial expressions. This even happened in Beast Wars, in which the character in question may not even have a mouth.
Taken to an extreme with Transformers Animated Shockwave, who has no facial features at all. Well, when he's not in Longarm Prime mode. All he has is a red circle as an optic sensor in the middle of his dark face, but it changes shape subtly according to his moods the way the other characters' optic sensors do.
Filmations GhostbustersBig Bad Prime Evil had a robotic skull for a face/helmet; despite the fact that it was apparently made of metal, it could cartoonishly change appearance for a variety of expressions (but then, since he's already a ghost, this could be a Justified Trope.)
Most characters in the BIONICLE movies have these.
The Reptar mask used in the Rugrats episode Reptar On Ice is like this, with the mouth moving very realistically and the eyes blinking and showing expression, even though the rubber head is larger than the man wearing it.
Duckman. His glasses don't change shape so much, but his his eyelids (and eyes) are apparently part of his glasses, while his eyebrows float above them. As his glasses sit about a third down his bill, one can plainly see the blank yellow expanse where his eyes should be, whether or not he's got them on.
It has been shown that the glasses contain his eyes even when he sleeps with them on the nightstand next to his bed - literally Blind Without 'Em.
Barely noticeable, but Photo Finish from My Little Pony: Friendship Is Magic and her oversized glasses certainly count. It's most noticeable during her interview with various newsponies about how she discovered Fluttershy as her next big star.
The Shadowbolts play it more straight.
Rainbow Dash's Shadowbolt costume, by extension.
In Wakfu, Nox's mask has shutters that act as eyelids. However, more often than not, we get the full unsettling effect of those unblinking glowing eyes.
Subtly done with Amon in The Legend of Korra where slight changes to his mask's mouth give a better representation of his thoughts.
Almost impossible to spot on Belphegor's mask, from the Belphegor TV series. His mask lacks a mouth and seems featureless, if slightly scowling. However, sometimes when he's upset the scowl seems to deepen a bit.
On Invader Zim it's hard to tell that Dib even wears glasses; not only do they adjust with his expression but the frame is apparently wire-thin and there are no temples connecting them with his ears. Zim's fake human eyes and GIR's eye cups also adjust with their expressions, which is presumably Justified as alien technology.
Death gets one Expressive Skull in the animated adaptations of the Discworld novels.
The Grim Reaper usually has expansive eyesockets (and probably magical levitating eyebrows and / or a deformable jawbone) whenever he is depicted as a character with personality rather than just a force of nature.
Many high-quality full-head silicone and latex masks (particularly those made by SPFX Masks, Composite Effects, Immortal Masks and Greyland Productions) are capable of having somewhat-to-hyper realistic mouth movement and facial expressions, bringing Latex Perfection to real life.