One of the most important aspects of human beings, overshadowing and enveloping all the specifics of behavior and development, is change. People change, people make changes, and people are changed by things. Human beings are not static; even an individual who stands perfectly still is still aging, still using up energy, and so on. People exist in a constant state of flux, adjusting to their environments and surroundings in order to survive and thrive. Simple changes lead up to larger organization and eventually technological cultural developments, and it is through change that humans (or any other life forms) became what they are.
Naturally, if you're trying to make believable people, this is an aspect that ought to be included. This doesn't just have to be in the present tense (i.e. change happening when the camera's on), but can also be reflected in a character's past or history. It doesn't have to be big changes, like personality development or standards, either. Even the changing of clothing is a "change", though a routine one. All the elements of the decision-making process are part of change, because making a decision is putting change into action. Essentially, though, acknowledging change, of whatever kind, serves to deepen the character's relationship to the environment they're in. It creates a cause for all their "effects": the nature of their personality or their physical appearance, the skills they possess, the knowledge they have, and so on.
The human body is a machine. It's built to adapt and thrive just as the human brain is. It's not as specialized as many other animals, but it has the capacity to improve, and it has internal mechanisms that require changes on some level or another. These changes can show, through visual evidence, a character's past and their status. When thinking about or visualizing the body, it's often best to assemble it piece by piece. So let's start from the top:
The human head is pretty important for a lot of different things, but in terms of visual design it's most important because it's a focal point for interaction. Therefore, the design of the face and its components will be the first thing we focus on. The face can represent several different things. It can be gaunt or chubby depending on the individual's weight and lifestyle. It can be unmarred or weathered depending on the hardships that the individual has faced in their life. It can be smooth or wrinkled based on their age or quality of life. The basic facial model is made up of a combination of bone structure and skin, and reflecting proper conditions allows even these basic aspects to tell a story through visual cues.
In addition, there are different things that can be added to the face through believable means that affect its visual profile. The most basic of these are facial hair and makeup, since these are things that are generally part of daily life. Showing facial hair and its growth might be justifiably considered a pain for artists or designers, but the representation of its growth and the character trimming it makes it feel more like real hair and less like something stuck onto the character. Similarly, makeup ought to be something that's applied, rather than being permanently attached to the character 24/7. Both of these things affect a character's face, so making them malleable rather than permanent allows a degree of logical changes within the realm of believability.
Hair has a combination of easily-malleable and difficult-to-change properties. The latter includes basic properties like color and texture, while the former reflects the fact that hair can be cut, trimmed, tied back, put up, and so on. Therefore, a combination of these two factors makes hair more relevant to a character's decision-making processes while still retaining a justifiably interesting aesthetic. The issue for believability is connecting the fashion to logical choices and processes rather than being something the artist thought looked good.
Probably the easiest and simplest way to alter hair is to tie it back or braid it. This can signify several different things depending on your perspective, but from a general utilitarian standpoint long hair being tied back means that it's not in your face and thus communicates a more no-nonsense approach. Short hair can convey the same message. The nature of the hair, even in such cases, is also variable; compare a stylized short hairdo to a simple buzzcut, or a quick, unkempt ponytail to a tight, professional one. The texture of the hair also conveys aspects of a character's life, depending on whether it's clean and loose or thick and raggy.
Not only simple or utilitarian hairstyles make commentary, of course. If a hairstyle is justifiably elaborate, it says something about the character and their willingness to spend time and effort shaping their hair into that form. Adding this element of believability turns it from a simple design choice to a visual cue just like any other hairstyle; without that element, it doesn't mean anything. If visible effort is a part of the universe, then a complex hairstyle can signify wealth, vanity, or just a willingness on the character's part to spend time on their appearance. There needs to be that element of effort in order for a complex hairstyle to be appreciable as part of a character rather than simply an artist doing what they think looks good.
Finally, it's important to convey the weight and nature of hair, even if that's fairly low-key. The consistency, thickness, and solidity of hair help make it feel more real, and what I see with a lot of artists is basically making the hair like some kind of glossy, solid mass. Hair flows, moves, and sways. If hair gets in your eyes, it's harder to see. It can be grabbed at or get caught on things. These simple things appeal to sensory concepts, just like many other "believable" materials, and if it's real to the characters it's more real to the audience.
Like the face, the body can be shaped by a character's lifestyle. Weight, muscle mass, skin tone, and skin consistency are all simple elements that, in their own ways, reflect where a character's from, what their life was like, and what they're capable of now. It's the difference between a scholar and a laborer, or a noble and a commoner. Like any other part of a character's appearance, a character's life makes visible changes on their body. Some parts of this are easy to reflect; tanning in general is pretty simple to understand, whether it's a farmer's tan or a beach tan. Weight is also pretty simple, at least when you're creating a divide between starving beggars and opulent aristocrats. In more modern contexts, the "weight = wealth" issue isn't nearly as common, but in any setting or situation where food is a rarity, the ability to be fat and unhealthy is something that most folks won't get away with.
Muscle development, on the other hand, is reasonably complex. I think this giant image that I'm linking right here says things a lot better than I can, but the basic lessons that should be taken from it is that muscle development isn't like an on-off switch, but is dependent on the cause of development and the locations being developed. More importantly, the body-builder physique (which some of us probably think of as being the peak of musculature) isn't necessarily the best physique possible when it comes to muscle development. However, both form and function have their uses in believable development, because they're just alternate routes by which a character has come to their current condition.
Most importantly to this topic, bodies change in the long term. To use a classic example from the children's novel "Holes", the main character's body is described as going from tubby and pale at the beginning to lean and tan from all his time in the desert. There are certainly many other examples, but the body is something that is influenced by environments and lifestyles. It's something that's more long-term than hair is, and it's not as directly controlled by the character, but it's a malleable element that can be used to show changes in a character's situation.
I've already done some articles on outfitting - whether it's clothes, armor, or gear - but the role of those things in this article is the fact that, underneath their clothes and gear, people are people. Humans are universal across any setting they're present in. Clothing and equipment, however, reflects their life as well, because it's a combination of available resources and needs that must be filled. Clothes need to be made from materials that are available, whether it's cloth, linen, fur, or some other material, and they need to fulfill some role for the characters who wear them.
Relatedly, a character's possessions ought to be considered logical items in their own right, not just part of an iconic outfit. Clothes can be changed and chosen depending on necessity and availability. The person wearing them is relatively constant, but clothes are the most easily altered element of a character's visual design. Like any other part of a character's design, ignoring cause-and-effect with regards to equipment and outfits makes them less meaningful in design terms. As I mentioned with Soul Calibur, a character who wears clothing that makes no sense for them (or wears the same outfit all the time) feels less like a character and more like an artist's plaything. In contrast, creating justified reasons for clothing choices makes a character feel more like someone who's actually making decisions, which allows the audience to better suspend their disbelief.
If there's a lesson I'd like you to take from this article, it would be this: people change. Things change. Situations change. Change happens all the time. If you can reflect that in your characters and your designs and your story choices, the world will feel more real and more developed. Everything that happens ought to be explainable and justifiable in-universe, and that doesn't mean you can't have cool things or interesting designs - just that they need to be as impressive to the characters as they're meant to be to the audience.