I don't exactly know what I was going for here, but I have done a few dialogue only fics in the past and wanted to challenge myself. This is the result of rambling, so hopefully it's alright. Some homophobic language and swearing included.
"I need to lose weight. Nurse said so."
"I don't think you're that fat."
"Well, news flash, I am. And when do you ever think?"
"She's stupid then. You're… big, sure, but you're not that fat."
"Says the bloody skinny rat… Oi, you just said I'm 'not that fat.' So you agree with her then?"
"I don't look like a rat!... And no, Dud! I think you look fine!"
"You don't fancy me or anything, do you? Cause that will make you a poof, and I don't like them."
"I ain't gay. Is your cousin?"
"He's your only cousin, right?"
"Yeah unfortunately, and he probably is. I bet all mag… erm freaks like him are."
"What, people at St Brutus's? You think all them criminals want to shag blokes?"
"I dunno! It don't matter whether Potter is a poof or not, my parents still hate him."
"They hate him cause he's potentially gay or cause he's a criminal?"
"He's a fucking burden on us! Wish he'd died with his parents…"
"Shit, D. Bit harsh?"
"Why do you care? You don't like him either… Unless you fancy him?"
"Told you, I'm not gay! And even Potter don't deserve to die in a car crash."
"Yeah, car crash… Anyway, I'm supposed to go and see Mr White. He's gonna show me around the gym."
"Hate him. He always makes me do more pushups than everyone else."
"I don't want to go."
"Why not? You scared?"
"Oh come on, Dud. Anyone says anything, you'll just pound them."
"It's not the words, I'm… It ain't the words, I'm scared of."
"Oh… you think they're going to laugh at you?"
"Well, duh! They already say stuff behind my back about what a whale I am and how my parents are feeding me to death! I… I want to lose the weight, I'm tired of being tired, you know?"
"You seem fine."
"Stop bullshitting, Piers! You seen the way I am after going up one flight of stairs! I can't bloody breathe…"
"You don't need to exercise. Surely you can just cut down on sweets?"
"I'm… I'm not eating any more sweets."
"Okay, that's a big change of heart. Any reason?"
"Something… happened. In the summer, when Potter left to go to school."
"Something with sweets?"
"Yeah, had a run in with Potter's mates. Don't worry, I gave one a bloody nose."
"Hah, now they know not to mess with you, Big D."
"Hmm. So yeah, I'm not eating sweets and I have to go gym. Nurse said she'll tell the Head if I don't. C-Can you come with me?"
"Er, I don't think that's a good idea…"
"Please? I don't want to go on my own! You already said I'm not fat, so I need the support! Don't make me ask again!"
"Alright, alright, just don't hit me again. My nose still hurts from yesterday."
"Hurry up, get off your arse, I should have been there ten minutes ago."
"Shit, Dudley! Mr White is going to kill you!"
"Good. At least then I don't have to go through all this pain to lose the weight. Why can't I just wear my own clothes to school? Schools in America do."
"Don't joke about dying, D. It ain't funny."
"Since when have you been all sensitive? Get up, we have to go."
"Fine, fine. By the way, I'm not joining in."
"Oh yes, you are. I ain't doing the exercises by myself."
"But Dud, I don't need to lose weight. Everyone always says I'm too skinny!"
"Lucky. So you gain muscle, I lose weight. It's fair… Oh damn, I just remembered what the Nurse told me to eat for dinner tonight."
"Is it bad?"
"Stop laughing! You're eating it too!"
"No, I'm not! Grapefruit is disgusting!"
"So's your face! You'll eat grapefruit with me tonight, or your mug will end up like Donovan's."
"The kid you hit last week? Second year?"
"Okay, I'll have grapefruit with you… Dud?"
"I think you'll smash the gym. Like, not literally, but-"
"Piers. Shut up. And cheers."
"No problem?... Bet when we go home, no one will laugh at you."
"Cause you'll have a new shape."
"What the hell you on about?"
"New body shape."
"You seriously sound like a faggot."
"I'm not! Just saying, you'll be ripped or something and get all of the girls."
"Oh, but Piers, you're always the one for me."
"Joking, idiot. I doubt I'll get muscles. I just need to lose enough weight to fit into my school trousers, I ain't motivated enough to do an entire body overhaul."
"Make you seem more intimidating."
"Yeah, more kids in the neighbourhood to be afraid of you."
"Piers, everyone is already afraid of me."
"Potter ain't. Not anymore, from the looks of it. If you're built like the Hulk, then even he'll piss himself from fear."
"Yeah… but I don't wanna imagine your cousin pissing himself."
"You're weird, what the fuck? Who thinks like that? But yeah, if I work hard at the gym, I'll get good results and show Potter who the real man of the house is."
"Yeah! You could beat him up next summer!"
"Er… I won't do that… He's crazy, remember? Never know what he'll do next."
"Yeah, he did turn our teacher's hair blue. Remember?"
"Yeah, I was there."
"Well, least you can tell Mr White you're gonna work hard at the gym. Hopefully stop him from screaming at you for being late."
"Urgh, I don't want to put all those hours in! Stupid weight loss. Can't I just get surgery and cut it all away?"
"Sick! You could give Potter all your loose skin as a Christmas present!"
"Fuck, Piers. That's gross. You been watching too many of those fucked up films again, haven't you?"
"So's your Mum."
"Oi, watch it."
"Didn't say nothing about her, did I?"
"Right, you want to go in first."
"Dudley, he's expecting you."
"It weren't a question. Go in, now!"
"Alright, alright. You owe me for this."
"I give you the gift of my friendship and this is what I get?"
"Dud, you asked me here, so don't spout that bullshit."
"You want a bloody lip? Move!"
"Ow! You don't have to pinch me!... Alright, Sir? Sorry me and Dudley are late, I had to wait for him to finish wanking in the toilets."
"OI! SHUT IT, RAT FACE! COME HERE! STAND STILL SO I CAN HIT YOU!"