In the TV series "Humans", you can get a feel for who is human and who is a synth, but it's very subtle. Also, I suspect that a given synth looks more human-like at a given time, such when Anita's adult mode is activated or she's hacked into.
This "Behind The Scenes" article mentions some acting differences, but I assume there must be prosthetics or CGI as well.
What differences in appearance between synths and humans are there in "Humans", and how is it achieved?
Major spoiler alert for season one of Humans, yada yada yada, read at your own risk if you haven't yet watched it.
From your question it seems you want an answer as to the in-universe and out-of-universe differences, such as use of props/makeup to make the actors embody the synths they portray.
The main physical difference between humans and (non-conscious) synths is the way they walk and talk- a manufactured (non-conscious) synth is the most likely to do this: they move in an almost seamless way, heads turning gracefully etc. It's best to study some footage of the way a synth moves, good examples are the synth that's handing out leaflets outside the station and the way he cocks his head and smiles mechanically at Laura.
Such movement is most notable in manufactured synths who were built to serve (real) humans; their movements are unnaturally well-timed and precise, unlike regular humans who walk
Out-of-universe, this was achieved by sending every actor who played a synth to a special "synth school". Notice that the article mentions that, due to subconscious mimicking, it was equally hard for the actors playing humans to retain their usual "sloppy" posture and movements.
Behaviour among the "conscious synths", who were made by David Elster with the intent to create machine consciousness.
Niska and Karen are the best examples of conscious synths who do not move as rigidly as the other conscious synths: Karen manages to trick both the audience and her police co-worker Pete Drummond into thinking she was a human. Similarly, Greg begins flirting with Niska when she poses as a human, even taking her back to his apartment... (See below for more info on synths [especially Niska] changing their appearances.)
[Karen during her life as a "human" police worker]
Another main difference is the green hue of manufactured synths.
Out-of-Universe, this was achieved by giving the actors who played synths green contact lenses.
'Persona Synthetics' clearly wanted to make it easier to tell the difference between a synth and a human, as demonstrated by the house-synth 'Odie' and this female synth from a 'Persona Synthetics' advert.
Many of David Elster's "conscious" synths also posess this trait, although it is unclear why, as the green exhibited is not a natural eye colour and would immediately reveal that the unit was a synth rather than a human. Interestingly, out of the conscious synths, Max, Fred, Mia/Anita and Niska have their eyes green full-time (Niska has to change her eye colour with blue contact lenses to make herself fit in).
[Niska's natural eye colour]
[Niska's eyes after she puts contact lenses in.]
... although Anita/Mia has blue eyes before she is attached to her "primary" user(s).
It is possible that David Elster created Laura with non-green eyes as she was made in Beatrice's image, although I'm not sure why he would make his conscious synths so conspicuous.
Many of the synths traits are based off the "Hubots" from the original Swedish series, Real Humans.
"Though they are designed to closely resemble humans, hubots are usually easy for humans to recognise as they have bright flawless skin, glossy hair and unnaturally bright (usually very blue or very green) eyes. All Hubots also have a USB-like port, in either the back of the neck or in their lower back, which is used for programming and data. All Hubots also have a USB-like port, in either the back of the neck or in their lower back, which is used for programming and data. The button to activate or de-power a hubot is located under the left armpit, as is a standard wall plug cord for recharging purposes."
The only instance we see as to the placement of the power socket of synths in (series one of) Humans is when Laura reveals to Pete that she is a synth by removing the skin on her left hip, and revealing a small round charging socket.
As with Real Humans, synths in Humans have blue "synth fluid", which we see leaking out of Odie and prepared to be given to Max when he needs a transplant, and Karen when she cuts her hand to make herself bleed for further proof that she's a synth.
Out-of-universe, there were probably prosthetics used to create fake skin, blue paint etc.
They did not use contact lenses (out of universe) in Humans. Here is the info on the VFX technique used according to Chris Fry, the series producer.
I looked into two options: contact lenses or doing it in VFX. My issue with contacts was that actors can only really wear them for eight hours at a time. We often had up to ten characters on set on any day, so there were clearly potential scheduling issues.
This led me to favor the VFX route. Doing it in post gave us much more freedom on the floor but also gave us creative control. It was important to retain the sense of realism: the eyes shouldn’t ever glow and the pupils had to contract and expand, the same as the actor’s human eyes. This would enable us to connect with the synth characters without having the barrier of a coloured contact lens.
The Flying Colour Company pitched to me and they were really in tune creatively with what we were trying to achieve. They gave us no restrictions; they simply said: “Shoot what you need to for the scene and the eyes are our problem”. This was exactly what we needed as I didn’t want another element to complicate the shooting days.
Dominic Thomson, creative director at TFCC, designed a specific kit for each of the synth eyes and tailored it to their look. An Autodesk Flame was used to create an image and texture-overlaying technique that would change eye colour while interacting with both the characters’ natural eye colour and the lighting of the scene.
Each character’s eyes would then be tracked throughout every scene and the overlay effect applied and refined in Flame. The team including Jaime Leonard completed up to 300 VFX shots per episode across all eight episodes.
