In recent history, humanity has been notorious for its obsession with fashion. Depending on the location and the type of character you're making, they might be up to fashion trends of the time, or they might wear a ordinary outfit. To determine this, figure out how flamboyant or look-obsessed a character might be. Some characters with no flamboyance won't follow the rules just for the sake of it and instead dress how they feel like, whereas characters who are more eccentric might pay closer attention to the trends since they believe it gives them more status just because they dress a certain way. In other words, a scientist will typically wear something practical no matter where they go (maybe in your fictional world they're recognizable, like a uniform) whereas a successful club owner might have acquired expensive tastes, thus dressing in a much more opulent style just for the sake of it.
Sometimes, like any other topic, we're uninformed of what the right call is, the 'right' outfit for a character. When designing my main protagonist, only three things remained the same between the concept of her design and the final design, which were her trench coat, silver headband, and her black-dark palette. It was actually my friend who suggested that, because she's a scientist, I should change her design from a generic sweater to have a white dress shirt and tie; so I chose blue to match her eye color. The trench coat itself changed from fully black to black with blue accents; she started wearing black rebel boots, slacks with belted pockets, and so much more about her design changed because I had a different type of artist take a look at the character design. Since he draws, he has a different perspective; a perspective that helped significantly with the overall character designs of the human world for me.
Now, I think that's enough about clothes; you get the idea. What about the characters' looks themselves? Well, you can make the argument that unless there is a specific reason for a character to look a specific way, it's best to just leave it up to the reader's imagination (and that can also apply to the fashion if you don't feel like crossing that detail). That being said, sometimes it helps to establish a description. A person who's tall and thin might much more easily get through narrow passageways than a short but muscular person. Does it matter to show disparity? If so, why? Every detail matters, and if you give the reader useless information, they're going to roll their eyes about it and forget about it. If their hair is brown, what does that symbolize? If their eyes are green? How much does ethnicity matter?
It's really difficult to make every single detail match, but if you've had a lot of time to make a book and then edit it a few times over, it's as good as it's going to get for now. That's also to say that your character design can change in future books. Surviving characters from previous novels might have a different character design than in the first book, especially if a lot of time has passed. Maybe each novel is standalone and you want to try out your different character design ideas on a brand new protagonist. After all, it's a fresh look, and it can really help differentiate one story from another just from the physical realm.
If you're writing a novel, you can also change specific details about the character if you think it's necessary between parts of the story. Clothes are the easiest, but maybe your character undergoes a physical transformation. If you're writing a contemporary book, maybe they're exercising to build muscle and athleticism or overeating and becoming overweight. In fantasy books, this can be as simple as them making a sacrifice and becoming more 'pretty' like Yennefer of Vengerberg (The Witcher); or the character might become a werewolf, a vampire, even a zombie because of corruption. In a more scientific realm, maybe you want to take inspiration from All Tomorrows and engage in genetic modifications (though they don't have to be as grotesque; they could improve the characters themselves to be stronger, have better senses, etc.). Or, maybe you're writing a horror book and engaging in body horror. Little by little, the protagonist would change throughout the book until they've metamorphosed into something else (or maybe they go back to normal, who knows?). Like any creative project, have fun with the design; do whatever you feel is necessary with it to further your story.
The last piece of character design might be the place they live, and potentially the most subtle piece of it. If your character lives in a mansion, by proxy their personality is going to be a lot different to someone who lives in a wooden shack. Their occupation might mirror where they live as well. Most of the time, you will not see a Lockheed Martin aerospace engineer as homeless; otherwise, there wouldn't be much incentive for such an intensive job. Granted, someone can be rich and unemployed if they live off the interest, but much more likely, a poor person who is unemployed is trying to get a job and struggles to live because they cannot support themselves. Furthermore, where a character lives is important.
A person who owns a house in Aspen, CO is probably orders of magnitude richer than someone who'd live in Winnemucca, NV. That's not to say that Winnemucca is necessarily a bad place, but it's a lot cheaper to live in that area than in Aspen. Granted, these two areas are completely unrelated, so if it's a story of opulence and rags, you'd likely choose the same city and split it based on the richer and poorer parts of the city. In Anchorage, AK, the Hillside neighborhood is the most expensive and prestigious. In contrast, Government Hill has a poverty rate of 48.5%, and Mountain View, a bordering neighborhood, ranks second with 38.2% of its population living in poverty. Compared to the ~$160,000 average annual income of a Hillside resident, this wealth gap in a single city (or metro area) will draw borders in a different way entirely.
To wrap everything up, small physical differences make a character's personality in some ways. Some people might be naturally terrifying to others; there'll be the type of person who gets upset because of it, and another who will use it to their advantage to manipulate their way. Even from what they own, what they value, how they express themselves through their look, everything has to be relevant to the story. Even if it's not actively a part in the story, as a background detail it has to insinuate something or paint a picture for it to be a good subtle detail. Like in a TV show, the reader wants to figure out plot twists before they happen, and they can only do that through context clues; the same is true with backstory and character design.
Thanks everyone for reading this far and tuning in! Three months from today, if all goes well, we'll have global distribution! I've been hard at work to ensure that my book will be available to a global market; not just the United States. I'm setting up an ebook format, hard cover, soft cover, and an audiobook. I hope you can find a copy in a format that you will enjoy, so I'm making the extra effort to expand printing and electronic operations. This next week up until late April will be catalogued by book promotions on other media. If you're tuning in for the first time, why not subscribe and stick around? It's free, and I write these out of my freetime every week.
Until we meet again,
Dustin