Physical needs include air, water, food, and sleep, and the other things our biologies need to survive. (The other 10 categories deal with emotional needs.)
A character meets her physical needs by breathing, eating and drinking, exercising, sleeping, and even evacuating (although I’d be careful about working that into a story).
And any situation or environment which denies these physical activities will threaten a physical need. The vacuum of outer space threatens a character’s need to breathe. A lack of food may prevent her from eating as she needs, just as feelings of low self-esteem may encourage her toward anorexia. A touchy social situation—or a really dirty public restroom—could also result in some awkward physical needs.
When a character’s physical needs go unmet, she experiences direct, physiological symptoms. If she doesn’t eat, for example, she will grow weak; her body will use energy stored in fat reserves; after those are exhausted, it will begin feeding on muscle and brain; and eventually, if she still doesn’t find nourishment, she will die. Denying other physical needs similarly cause her health to deteriorate, and searching on the web frequently reveals real-life cases on which you can pattern your character’s experience.
The classic extreme has enchanted each of us at least once: the pregnant woman wrestling with raging pregnancy hormones, who not only craves bizarre food combinations but also cannot seem to make up her mind what she wants to eat. You don’t have to explain why she’s ravenous for deep-fried cheese with honey mustard one second, and pineapples and potato salad the next. All you have to do is point out that she’s pregnant, and all of us who have either had a baby or known a loved one who has, we all immediately understand.
Most biological activities also have emotional components. Sometimes a character eats, not just because she’s hungry, but because she finds comfort in food. She’s using food to fulfill some emotional need, as we’ve all turned to comfort food at some point, to relieve stress or after breaking up with a romantic partner. In the extreme, this behavior can grow into a dysfunction, in which the character eats—and feels hungry—not for nourishment, but as a substitute for something else, such as emotional intimacy.
Biological activities also commonly tie indirectly with emotional needs. Eating and drinking are obvious examples. We eat and drink not only for nourishment, but also as a social activity (which supports our senses of security and community) and for taste sensation (which ties in with our spiritual needs).
Some people include sex as a physical need, because sex itself is a biological function, a physical act, like eating or peeing. And this physical act is necessary to keep the species going, so we are pre-programmed to perform that physical act. (All of us have felt horny on occasion without prompting or explanation.) So as a physical act, you could include sex as a physical need. However, if a character doesn’t have sex, he won’t die. Moreover, sex’s emotional implications are usually more significant than its physical characteristics, a fact all romance writers should keep in mind. Sex stands as one of the most powerful and profound human activities, because we link it with a wide variety of deep emotional needs, including our need for attention, emotional and physical intimacy, family stability and security, status, and even spiritual growth.
While physical needs may be the most basic, almost clichés, don’t shortchange them in your characters, because they are core human needs that can add sympathy for your characters, and they’re related to the emotional needs that we usually think of when we think of “character story.”