While there are different kinds of discomfort a character could suffer, this entry focuses on physical pain, which is most often associated with an injury or illness. 

Pale or blotchy skin
Glassy eyes with visible redness
A pained expression
Lips pressed tight, the mouth appearing smaller
An audible pained breath (at moving too fast, accidentally touching an injury, etc.)
The character grinding their teeth
Sweat beading on the character's forehead and causing their face to appear slick
A bent back and caved-in chest
Shoulders hunched, drawn in close to the body
Walking with stiff movements
A visible trembling in the limbs
A weakened grasp
Probing, rubbing, or grabbing at the affected area
Wincing
Flinching when touched
Leaning back and grimacing
Expelling a grunt or hiss upon exertion
Shaking or flapping a limb that's been injured (when the pain is minor)
Asking for aid (to move, get something, call for help, etc.)
Hobbling about; taking tentative steps
The injury site swelling up
The skin around a wound going red
Crying
Taking a pain medication
Breaths that saw in and out
Taking deliberate deep breaths
Flaring nostrils
Trying to sleep the pain away
Gripping people or nearby objects for support
Cradling the wounded body part
The character's voice sounding pinched or strained
Speaking in short, clipped phrases
Answering questions with head nods and shakes
Screaming, moaning, or groaning
Rocking back and forth
Panting
Repeating the same phrase over and over
Squeezing the eyes shut; refusing to look
Squirming in discomfort
Arching the back

Various pain sensations (sharp, slicing, coming in waves, pulsing, etc.)
The affected skin or joint feeling tight as inflammation sets in
The wounded area feeling warm or hot to the touch
Nausea
Hyperventilating
The character feeling lightheaded at the sight of blood
Starbursts behind their eyelids
Tight muscles
Dizziness; feeling faint
Cramping muscles
Feeling cold or feverish (from infection)
A dry or coppery taste in their mouth
Trembling hands and feet
Uncontrolled shivering
Feeling an overwhelming need to sit or lie down
A tight chest; the character finding it hard to catch their breath
Being overly sensitive to stimuli (light, sound, pressure, etc.)

Engaging in mental self-talk (to stay calm, to remind themselves the pain will pass, etc.)
Short-temperedness
Trying not to think about the pain
Conjuring up memories and experiences that were worse than this
Looking for signs that the pain is waning
A sense of time distortion (losing track of time, feeling it's passing more slowly, etc.)
Worrying about worst-case scenarios (if the pain is new and the source is unclear)
Worrying about people in the character's charge or others who might also be hurt
The mind going over possible negative effects of the injury or illness—if it will impact an upcoming trip, for instance
A single-minded focus on relieving the pain
Masking the pain in certain situations—so children don't panic, for example, or to keep an enemy from knowing the true extent of the character's injuries
Finding it difficult to concentrate, problem solve, or remember things
Being unable to answer questions or engage with others
Panic

A haggard face
Dark hollows under the eyes
Passing out
Medication dependency or addiction
Personality shifts
Begging for the pain to end
Depression
Asking for death
Researching, planning, or attempting suicide

The character clenching their jaw
Lips pressing tight and becoming bloodless
Smiling quickly and waving off the pain
The character lying down or sitting to avoid showing weakness
Remaining perfectly still
Avoiding talking to others
Cradling the injured area close to the body to protect it
Turning away from others to hide their expression or the injury
Clenched hands
Hands bunching up a blanket or clutching at clothing
Wrapping the injury to hide how bad it is
Finding a way to be alone to deal with the pain in private

Ache, throb, burn, tremor, sweat, twitch, hurt, sting, stiffen, moan, grit, bite, keen, cry, tear, groan, scream, stab, erupt, prick, writhe, wail, cramp, chafe, rub, agonize, pinch, grate, wince, weep, heave, faint, blister, redden, fester, bruise, cut, slice, support, lean, drag, crumple, hobble, shake, infect, tremble

ANGER, ANGUISH, ANXIETY, APPALLED, APPREHENSION, DENIAL, DESPERATION, DETERMINATION, DEVASTATION, DISAPPOINTMENT, DREAD, FEAR, FLUSTERED, FRUSTRATION, HYSTERIA, IMPATIENCE, IRRITATION, MOODINESS, OVERWHELMED, PANIC, RAGE, REGRET, RESIGNATION, SELF-PITY, UNCERTAINTY

Excelling in a sporting event
Being patient and understanding with loved ones
Being viewed as strong and invincible
Engaging in a physical activity that requires a lot of movement or stamina
Traveling a long distance
Serving or protecting others
Staying alert and awake

Having to rely on someone they don't like or trust
The character being responsible for someone else while they're struggling with their pain
Experiencing pain in a remote place without treatment resources
An environmental danger forcing the character to relocate despite their discomfort
Having to make an important decision while their brain is fogged by pain
Having to push through the pain (in a race or to escape danger, for example)

Everyone responds to pain differently; some express it freely while others will try to downplay it. When writing about your character's response, think about their pain tolerance and their personality, as well as the circumstances surrounding their discomfort.