Do you add pauses? Do you add interjection? Which ones? Do you misspell certain words? What are some of the tricks writers use? I don't want to say it in the descriptive passage. I want people to feel it while they're reading the character's dialogue. I want them to really feel the pain.
It's hyper-specific to a character. But here are some things I use. Note that the non-verbal cues will hit home harder when coupled with verbal ones.
Some will refuse to talk, not trusting their voice. With these characters I try to offer visual cues: profuse sweating, grimacing, 'pained' look, balling an relaxing hand, squaring jaw, or the well-placed unintelligible scream.
Swearing profusely. This is more striking with characters that don't swear.
Sharp inhales of breath (doesn't work with injured ribs/chest).
Being irritable, withdrawn, sullen. 'Needing a drink'.
Voice becoming haggard, shouting.
Suddenly becoming violent.
Becoming confused, forgetful, unable to focus (the pain is all they can process, and they wish to the gods to forget that too).
Otherwise unexplained actions. Suddenly needing to walk, suddenly needing to stop walking. Curling up into a fetal position (more common with extreme pain). Biting/sucking on finger (the sucking reflex we have as babies doesn't fade, it's just muted from disuse).
For other possibilities look into pain scales. (note: link is to one site, but there a many out there.
As requested, an example. Again, note that this is character specific, pain type specific, and injury or pain source specific. I'm a writer, but I was also a nurse, so I have a LOT of experience with people in pain.
Hmm, let's see. Injury: bruised ribs (incurred less than a day, so fresh-ish, but not as bad as initial injury, bet on a 4-5 on the pain scale detailed in the link above). Character: somewhat stoic. Scene, father (injured) talking to his daughter (1st person narrator).
Dad sits up in his hospital bed. His hair is dishevelled, his breathing even but shallow. It's the look in his eyes, though. That drawn look and the subtle frown even as he smiles.
"Hey." He coughs, his lips tightening against his suddenly bared teeth as tears well up in both our eyes. He takes a purposefully slow and deep breath, as if working up the courage to continue. "Hey, sweetie. When'd," he swallows as slow and careful as he can, another breath, shallower this time, "you. Get here?"
Things to take note of. Bruised ribs hurt. Not as bad as fractured or broken ribs, but they hurt all the same. And breathing with our chest causes ribs to move and do all the things you don't want. Speaking is a challenge with pain management with injured ribs. After a few days it becomes more 'natural' to speak with the altered speech pattern, so you learn to manage your breathing differently (or many patients I dealt with did this, consciously or otherwise).
If you are in pain, you will have a hard time concentrating, which can make a conversation a bit disjointed.
You will pause, catch your breath, maybe close your eyes.
You will stumble over words, stutter, put out the wrong word, correct yourself, maybe forget the word you're looking for. Stop. Shake the head. Move on.
Syllables may break a part. One. Word. At. A. Time. Then a blast of words all together in a rush with a sigh at the effort of it all.
Punctuate with damn, God, oh, a sharp intake of breath. The rhythm of speaking. Is off. Off. Can't really. Words aren't clear. Doesn't matter, the order, doesn't matter. Repeat yourself. Breathe. Start the phrase on a new word. Leave out everything but the core important part. You won't notice if your listener doesn't understand.
Questions don't matter. The conversation is what you need to convey. Eyes down. Other people are a fog. You only hope that your words reach them. But you don't know. You continue speaking because you don't know if you got through.
I shorten the phrases. One of my characters is in intense pain (she was gut shot) which makes it difficult for her to breathe, so her words are spaced, her sentences short. No unnecessary words.
I add a few extra dashes to emphasize the timing of what she says. Words come out in a rush, grouped differently and she is unsure if she will be heard properly as she must barely whisper.
Her internal monologue changes, thoughts of pain, hope of relief and wondering if she made a massive blunder. Was her companion worth the sacrifice?
She believes herself to be dying, so makes no unnecessary statements. Every word hurts.
It is a combination of factors but let yourself become that character, imagine the situation. If the injury is to a limb, your character might be reluctant to move and possibly sardonic, but breathing itself would not hurt, so speech would be less impaired.
That said, when someone is in pain it can alter them temporarily. A person with a wild sense of humour might not be so quick to quip when injured. Conversely, they could go the Mercutio route and joke about their wound until they die.
One of my characters becomes irritable when he is injured and bleeding. Those who know him understand this and know to either intervene if it is serious or give him space if it is not.
The reaction to pain is unique to the individual so I have characters react differently since they are different people. When my MC gets shot, he is livid in part because he was shot by an idiot and his pride has taken a more serious blow than his body.
When my secondary protagonist gets shot, she is at first fearful that she wasted a gallant gesture on someone she thought was deserving, but suddenly doubts. Her doubts fade as her companion proves himself to her, and she is content. She just wishes he would be honest and tell her she’s dying.
Edit requested by OP
Jack roused himself, the vicious bastards had him in the interrogation room now. Shackled. The trip in the back of the cruiser had made things worse. Hurts to breathe - might have some broken ribs. Law suit, let there be a witness. Everyone has a cellphone - hadn’t noticed any but he’d been distracted.
“Jack, why did you do it?”
Squinting through the pain, he gasped, “Do what?”
“We know you were there. Just tell me what happened. Resisting arrest wasn’t the smart move. You are not a fool.”
“I know you were there. Do you really think we pulled you over for a speeding ticket?”
“Listen, Davies, there are some guys in this precinct who don’t like you much. You noticed, right? Cooperate now.”
“You stu pid bastard. Look at my ID. Wrong man. Name’s Wilson.”