One disadvantage to fictional characters is that they can technically survive anything. It may not always make sense, but they can. They don’t whine when they’re hungry. They don’t pass out from exhaustion. They don’t get weak from thirst or disease. You can riddle them with bullets or make them into swiss cheese with swords. They can fall from extreme heights and only suffer a scratch. They can... Well, you get the idea.
That means it’s up to the author to remember the basic needs of characters that allow them to appear human with human limitations. It’s up to us to feed, water, and care for them. Very few characters in fiction can go without having physical, emotional, or mental needs met, so it’s a sure thing that when you write you should keep the following in mind:
Food: What does your character eat? How often? How does this impact his overall health and ability to perform in your story?
Water: Or some other fluid. Hydration is essential, and it doesn’t take long to become dehydrated and suffer some pretty severe side-effects like weakness, confusion, fever, heart palpitations, and within just a few days, death.
Temperature Control: What’s the environment like? Is it hot? Cold? How does your character cope with the temperature? How does it impact his ability to perform heroic deeds?
Elimination: Gross as it may be to some, it has to happen. No, you don’t have to include it in a story, but keep it in the back of your mind. Especially if a character is in a situation where he can’t relieve himself, it’s going to become an issue.
Illness and Disease: Most stories require putting the hero under a lot of stress to keep them interesting. Stress weakens the body and make it more susceptible to sickness. Keep that in mind, especially if your hero is spending cold nights outside leading a rebellion.
Sleep: This one is easily overlooked. Sleep is crucial for good health, and a character can go only so long without it. And the longer sleep is postponed, the greater toll its lack will have on the body, mind, and psyche. Even if a character catches sleep, is it good sleep? Is he getting his solid eight hours? Is his sleep interrupted frequently? Does he have insomnia worrying about what you’re doing to his life?
The nice thing about all of these is that, when you keep them in mind, they walk hand in hand with building tension. If your hero hasn’t slept in three days, that car chase is a lot more tense. If he hasn’t eaten since yesterday morning, that duel suddenly looks a lot more challenging. What if he faints in the middle of it? If he’s cold and struggling against hyperthermia, rescuing his daughter becomes a far more daunting task.
On the other hand, don’t be a slave to these needs. You don’t need and shouldn’t describe every meal, bathroom break, or how many hours a character slept. That gets tedious. The same rules apply as with all other aspects of fiction: include only that which is essential to the story. To some extent, a reader will assume your characters are eating, sleeping, and handling their other basic needs, but give the subject some attention to add believability to your story and to increase tension.