When it comes to creating characters, I've discussed ways that an environment can shape an individual - not just soldiers, but any human being. There is a tendency to make characters sort of exist in a static form, rather than a product of different factors and statuses. This can manifest as never looking different, never dressing different, never developing different beliefs, and so on. In contrast, things like character development and design development can help a character feel more real, and make events seem like they're having some impact on the character.
This doesn't need to be complex, though. You don't need to be a psychologist, a historian, or a paranormal expert to make a believable character. In essence, a character is ostensibly a human like you - different in terms of upbringing, values, and experiences, but still fundamentally similar. So here's a simple thing you can do to flesh out a character: plan out their day, from when they wake up to when they go to sleep. Include things like work, meals, and recreation. Think about your own life and the things you have to do every day, and then apply them to the character and the world they live in.
What does this accomplish? It establishes a lot of detail, including things like diet, hygiene, sleep habits, and living conditions. When discussing armor, I said that writers should ask themselves questions like "how would it feel", "how much does it weigh", "how easy is it to don", and so on. I brought up a similar concept when discussing buildings and cities: does a given area have all the facilities necessary to fulfill the inhabitants' needs? This is meant to accomplish a similar thing: to bring up the kind of questions that people can relate to, and to call on simple concepts that flesh a character out. We can sort these out into a few different categories:
Food and Diet
What does the character eat? Where does this food come from - nearby farms, supply trains, or a larger market? What food would be considered common, and what food would be considered a rare treat or delicacy? How does their diet reflect on their character - are they fat despite having little to eat, or thin despite an abundance? Where do they eat (i.e. do they have a specific cafeteria to go to)? Do they eat alone or with friends and comrades?
Hygiene and Health
What is the character's usual routine with regards to hygiene? How do they shave? How do they clean their teeth? How do they bathe? If a character is incapable of cleaning a specific section, that should reasonably reflect on their character (unless you find it distasteful, of course). What sort of minor ailments might the character have to deal with, and how would they remedy them? If a character is wounded or crippled, how do they live their everyday life?
Appearance and Equipment
What sort of clothes does the character own? How do they decide what to wear on a given day? Take a brief moment to imagine them getting dressed, setting their hair, etc. (this will avoid travesties such as this and this). What daily rituals do they have with regards to "sprucing up" their appearance? If a commonly-assumed thing (like "applying makeup" for a woman) isn't present or common, their appearance should change to reflect that. If their gear includes armor or a suit of some kind, how do they don it and how long does it take? How do they clean their clothes and maintain their other equipment? Where did they obtain their equipment from? When they're on the move, how do they carry their equipment? If they're not carrying all of their equipment, where do they keep the rest of it?
Work and Experience
What does the character do as their "day job"? This does not necessarily imply a mundane preoccupation, but instead reflects on how they spend the majority of their time and how they bring in income. How does this time reflect on their skills - remember that, since this takes up most of their schedule, it's going to influence their capabilities and knowledge. Learn about the details of their job, and what it entails. Try to avoid "handwaving" this time period - understanding their duties and schedule will help you conceptualize their job, and thus their characterization. What sort of role does their job play in the community? How much money do they bring in, and how does it affect their holdings? What duties do they have outside of their immediate profession - i.e. cleaning the house, taking care of children, and so on.
Free Time and Socialization
What does the character do when they're not working? How much time do they actually have that's "not working"? What do they do for recreation? Who do they socialize with? What do they do with friends? What do they have to do when they're alone? How does the flow of information affect their understanding of the world? Where does their moral compass come from? Where do they get news from - gossip, town criers, local papers, or mass media? How do they react to mundane situations (i.e. stuff like "stealing from an employer" rather than rarer decisions like "do you kill the bandit")?
With this sort of information in hand, think about your own life and your own schedule. Think about an average day for you, and then replace the concepts that are specific to you with the questions you have just answered. Here are some examples to get you started:
Daily Life of a Peasant
Daily Life of a Knight
Daily Life of a Nun
Daily Life of a Roman Soldier (book link)
Daily Life of an Ancient Egyptian
Daily Life of a US Army Soldier
Daily Rituals of a Space Marine
What this kind of exercise accomplishes is making the character feel more "real". One issue with fictional characters is that, really, they only exist in brief windows of excitement - the audience can't be expected to hang out with them for all the boring parts of their life. And yet, it is the "boring parts" that shape who they are, what they enjoy, what they know, and what they think about things. Their character doesn't arise from nothingness - it's a reflection of their upbringing, their environment, and so on. By understanding how the character's basic needs and requirements are similar to, and different from, your own, you can make a more believable individual.
Now, you might say that this is the kind of thing that bogs a story down - readers don't need to know about every detail, players don't need to manually clean every strap of their armor, etc. While this is certainly true, this (and many other parts of believability) are not about directly shoving this into the limelight. Instead, it's an underlying logic that affects the way the character is depicted and portrayed without having to necessarily be shown. Not that I am saying you can't show it, as naturally a well-done "mundane" scene should cause the audience to identify more with the basic aspects of the character's life.
In essence, the goal here is to make the character seem like a human being by connecting them to all the basic concepts of "humanity". People have a basic set of needs - food, shelter, socialization - and appealing to those concepts should allow the character to transcend issues of setting and culture and create empathy within the audience.
So, To Sum Up:
1) Creating a "routine schedule" for a character can help you understand and display the kinds of things that their life entails outside of your specific story.
2) It allows you to use a logical setting in order to justify where things like food, drink, and information come from - if your setting makes sense, every need will have a reasonable source.
3) It also allows you to draw upon your own personal experience in terms of basic character design, and then see how that basic concept changes when you account for the character's different perspective.