I have a shapeshifting character who isn’t human but pretends to be one, but people who look at them notice something is wrong, but can’t quite place it. What kind of features could I give them that make them look slightly less human? Currently, they are described as having a normal profile and face, if just very sharp and angled with a wide mouth.
The uncanny valley is an interesting phenomena discovered by Masahiro Mori, a professor who made realistic robots. Generally, the more something looks like a human face, the more people identify it as resembling a human face, but at a certain point (between 75% and 90%) people stop identifying it as a human face. The reason for this is a psychological one - people notice that the face they're looking at isn't human, but because of how similar it is to human, they can't tell why. Most people, after all, don't know how a human skull looks or what angles are acceptable for cheekbones, or the eye placement or anything of the sort. But the subconscious does - it has seen thousands upon thousands of human faces, so it knows something is wrong.
This clash between conscious recognition and subconscious rejecting creates the uncanny valley effect - people find the face "uncanny" and get completely creeped out by it. So, to make your humanoid character "look less human", just sink them into the uncanny valley so that everyone who sees them knows that there's something wrong and inhuman about their face, they just can't quite put their finger on it.
It doesn't blink. Unless it knows it is being observed, because a very little kid once asked it why it did not blink. It had not noticed blinking before then. Now it tries to remember to blink if it thinks it is being observed but it is easy to forget if it is concentrating on something else.
Its face does not move unless the shifter moves it on purpose. Again this is something that it figured out and now incorporates into its mimicry. If it is unaware it lets that part go.
It does not breathe. Unless it needs air to talk. This one is subtle and the shifter has not figured it out yet. In between utterances it does not take another breath. If it is exercising vigorously it does not increase its respiratory rate.
All of these things lend themselves to written narrative because as opposed to a subtly different characteristic that is hard to convey with prose, these are all actions (or the lack thereof) which are much easier to convey with prose.
It’s too perfect.
A major breakthrough for animation was the addition of imperfections. Before high resolution texture packs allowed for the addition of skin flaws (and before people realised the flaws were essential) animated characters simply looked fake. Humans simply aren’t perfect, no matter how hard we try there is always asymmetry, blemishes, spots and pores and dimples etc etc etc.
Not so your mimic. Their face is perfectly symmetrical. They have no freckles or wrinkles. Their perfectly blond hair is always perfectly done, even after walking through storms. Their fingernails are perfectly manicured and smooth and both eyes are a perfect uniform blue. The overall effect is like someone has tried to paint a human without actually seeing one. And real human seeing such a facsimile will immediately spot it as fake while the mimic will have no idea what it’s doing wrong.
After all, everything is perfect, right??
When I watched Downsizing, something felt really wrong in the scenes where downsized and regular people were interacting, but it was so hard to put my finger on it. I think it was because subconsciously I expected the fluid mechanics of air, gravity, and so on, to have created differences in motion between large and small people. Different wobbles in clothes and skin, different motion of hair, different difficulty of waving one's hands around.
When you move your leg, it doesn't stay the same shape. If you slow down a video, you see that the bone sort of drags the muscle and fat behind it, then they catch up and overshoot, then there's a bit of a wobble before they settle down. This motion pattern is highly dependent on the density and relative position of those tissues. There are also tiny ripples created by air resistance on the skin and skin-hairs, which are dependent on the stiffness and viscosity and density within the skin, coupled with the viscosity and density of the air.
The way your cheeks jiggle when you turn your head, the subtle wobbles in your face when you talk and laugh, all depend on the arrangement of things with different densities and viscosities (bone, fat, skin, muscle.)
Good shapeshifters would presumably have some sort of skeleton-like internal scaffold to help them be convincing, both with macro motions (like moving limbs) and fine motor skills (because they don't involve a lot of fast movement or air resistance). But it would be unfeasible to emulate a human's internals to enough accuracy to get the waves and wobbles right. If you're up close and see the way their skin undulates your brain will notice that the movements keep violating its expectations. But you couldn't put your finger on it.
Clothes and accessories
Their clothes merge with their skin. Only having observed from a distance and out of doors, they don't realise that clothes and accessories are not part of the human. Or, more likely, it's simply too difficult to shape-shift clothing on top of a body - there must be a join somewhere but where?
They will be caught if they ever have to undergo any kind of search or medical investigation.
Customs officer: Can I have your bag for a moment ma'am?
Shapeshifter: Erm ... I'm afraid it's er glued to my hand. I'll have to hold on to it while you look"
Customs officer: Not a problem. May we just check your shoes?
Shapeshifter: Umm ...
Customs officer: Glued?
Shape shifter: Yes.
I had a friend in high-school who didn't have any eyebrows (there are quite a few reasons people might no longer be able to grow their eyebrows). Whenever he met anyone new and actually took the time to talk with them face to face, there would be a moment when the new person would just stare at his face slightly confused. You could see that they were thinking "There is something off and I just can't place it." and then he would say "I have no eyebrows." And you could see realization slam into the other person's face. "That's it! That's what is off!" Afterwards, the conversation would continue as normal, but that awkward moment would ALWAYS occur when he met anyone new.
It isn't really enough to make it clear the character isn't human. After all, many people have conditions where they no longer grow their eyebrows. So you may want to add other traits. But from what I've seen, this is enough to give a moment of "Something is off and I just can't place it."
They sense the world differently. They see and hear in a different range of light/sound frequencies and amplitudes, they recognize a different set of smells/tastes through quite different sensory organs/mechanisms.
- they can only see red as we do, but some more in the near infrared spectrum. They can never get the colors right
- they can hear dog-whistle frequencies and are irritated by it
- Some of the most delicious food we have is quite disgusting to them (this even happens across human cultures). Have fun exploring how they reflexively react to something they find "disgusting".
- They can smell and recognize an object by extending an appendage (which for now looks like a human hand) to within a few centimeters of it.
- They hear in infrasonic frequencies, and try to imitate the human heartbeat/blood flow they hear to an abnormal volume that unnerves real humans
Imitating the full set of human biochemistry, metabolism and system functions is quite impossible for them. So maybe:
- They don't sweat. They vent through pores when overheated. How do they try to hide it?
- They smell different due to their biochemistry. How noticeable is it?
- They may imitate "eating" something but they do not actually digest it. How do they handle the food they ingested?
- They have a different mode of getting rid of their waste products. Does that sit well with human toilets?
Right now they're giving off vibes to others which could be a trait other humans recognize subconsciously that there isn't something right about your shapeshifter because it's not their species but obviously you want this to be more.
Voice - this shapeshifter is impersonating a human their vocal cords might fail or warble or do something they'll have to explain to others as a voice disorder. If you want you can have them claim a real disorder but then have them meet someone who actually has the disorder who knows they're lying or at least uninformed about what they claim to have. This way readers who won't think what your shapeshifter has is true symptoms of whatever you choose to go with.
Eyes - they could have collided pupils or a unique tone only to them (like vintage rose) or dusty (insert color) or you could split the color tone of the iris in half you can be complimentary or not or have one eye color split while the other is solid.
Skin - they could have patches of harsh skin that patchwork the smooth overall they claim or others assume this to be a skin disorder you can do anything here have rock hard patches or bark looking patches, or else switch skin tones or else give their skin a non-smoothness that pushes up from underneath it people can see something is up they could touch them and feel the under texture but again everyone might assume skin condition.
Hair - their eyebrows and hair have streaks of (insert other color here) it's not dyed its a biproduct of the presentation only a few strands have this color like the overall hair is black but streaked with auburn brown the odd is that it's also like this on the eyebrows no one would choose to dye this area in that way. Yes humans would place this again as birth add on or odd dye choice but the point is the shapeshifter knows this isn't as he/she intended it to go.
I'd personally go with don't describe the shapeshifter in heightened detail while in the human form. Allow others to imagine what is odd about them it makes it more fun for the reader.
Describe it as "just uncanny" for everyone but a genius.
The point about Uncanny Valley from @Halfthawed is really good, but describing it in a cool way is not easy. My suggestion is, make a small character, either very curious, like a child, or very smart and watchful, like a detective.
Make this guy realize some of the bits in @Mio's answer, and either narrate him thinking about it by himself or writing somewhere, something like (if it's a detective character): "He doesn't blink nor breaths often enough, when he smiles, just he's mouth changes, even a sinister smile would be less odd. I don't think anyone else realizes those details, but it's clear to anyone that there's something off about him, something... Uncanny, they just can't place it."
Just as Black Mirror does in the episode "Be Right Back", where a woman replaces her dead boyfriend with a robot, there very subtle things that make him less human, perhaps you could watch the episode and get some inspiration?
Things like not needing to: breath, sleep, eat, shower
Perhaps go with the approach that "it's weird they never ..." instead of a direct "There's something strange with their face"
A couple different ideas:
They all use the exact same template. Since humans look roughly alike to them they don't recognize how unusual many truely identical humans are.
Have them often misgender people by accident, it's generally difficult to recognize gender in other species and English uses gendered words all over the place so it would be difficult to hide the fact you get it wrong a lot.
Watch how some mimes can convince you they are carrying something heavy or light. Maybe their mass distribution is off so it always looks a little like they are doing the classic mime leadfoot routine to a degree.
When their own mass is different than the body they want to take the form of they just scale it linearly to get the mass to match. so they might have a child's build with an adult height. Or the proportions of a body builder just scaled down evenly. Since human heads are generally of similar size and vary less than weight, scaling everything equally would end up with an odd head size relative to body. Enough to seem a little odd at least.
They most definitely will not see the same spectrum range as humans, so they will likely not color match anything properly and if their power changes their skin tone based on their perceptions i'd expect it to be off to some degree. Even if they get a matching base color down, humans are not all one color across their body, for instance they may end up blushing with a blue tinge when they try to replicate it because they are red-blue colorblind.
You can make it so that they breathe significantly slower, or have no pulse, or don't sweat, or perhaps rarely or never eat or drink. They never get sunburnt or tan, they have no smell, or have seemingly no emotion. Perhaps their ears are shaped differently, or they lack fine details on their skin.
If you want specific tangible factors regarding the uncanny valley, you should look into videos for animators about animating real people. There's probably a bunch of stuff in there.
One thing I remember is something called sub-surface scatter where light penetrates the skin a certain depth and gets scattered back out. I think this is noticeable around your ears. Without it, you don't look quite right. I think it makes you look like wax or plastic if you don't have that. I'm sure there's other stuff but I'm not an animator so I don't know what it is.