When writing a novel, how can a character be developed well, but QUICKLY?

Engaging in conversation that also shows something about their intelligence, personality, wit (or lack thereof); and

“Well, maybe if you’d wear more clothes, they wouldn’t feel so cold. Hussy!”

“Damn straight. So get to it! This time next week, I want to hear some moans coming through that wall.”

What would happen if the US government told the British government in no uncertain terms all RAF bases with USAF personnel now must follow the Constitution and us law, and if the UK tried to defy this, the US military would directly attack the UK?

“I don’t know. Partying. Going to a pub. Anything besides sitting on the couch reading…” She squinted. “What the hell are you reading?”

“I’ll put the kettle on.”

“I’m glad my sex life is so entertaining.”

My boyfriend wants to break up over too many petty arguments. To me, they are molehills because I truly love him & don't really think twice about them. If he loved me would he work through it?

Create a context between this character and other characters.

“Claire! Why are you still up?”

“But they’re cold!”

Why do flat-Earthers think the Earth is flat?

“Nary a cute boy in sight.”

“Yes way. It’s washing itself under the street light. Uh-oh, I think it spotted me. It knows I’m watching it. I swear it’s looking at me.”

“They are! He broke the rules of the boarding house by petting this character while she was in cat form, so they invoke the ancient rules of single combat via ping-pong, and—”

Is it possible to revive a dead person in real life with black magic?

“No, about the cat. You don’t need a cat. You remember what happened to your spider plant, right?”

May pushed Claire’s feet away. Claire rose to peer out the window. “Huh. It’s still there.”

“You need some tea!”

What does 'Whose flesh is like the flesh of donkeys’ mean (Ezekiel 23:20)?

“It’s not looking at you.”

“I need to do laundry.”

“I know! That’s why I’m putting them under you!”

© you're so funny!

“You know what? Never mind,” May said. “I am way, way too drunk to be having this conversation.”

“Hang on, are they playing ping-pong?”

“Tart!”

Why do siblings (or other close relatives) stop visiting each other as they grow older? Why does this happen with so many people nowadays?

“It’s a cat. All cats are weird.” May sipped from her mug, inhaling the warmth. She closed her eyes. The room spun. She opened them again. “Ugh. I think I drank too much.”

“I try not to, but thank you for reminding me. I know I don’t need a cat. I don’t want a cat. What would I do with a cat?”

May studied the black and white comic panels. “Oh, my. She looks…anatomically implausible. What is she doing to that poor man? Wait, are those cat ears?”

What role did China play in the signing of the "Beijing Declaration" between Fatah and Hamas? What other information is worth noting for talks among Palestinian factions in Beijing?

“May! You’re home late! Early, I mean. Well, I mean, it’s early in the morning, but you’re home before I expected. Er, after. Before?”

After Eunice and I finished London Under Veil, I entered the first chapter in a contest at a convention where you could submit something and have it critiqued by a professional book agent.

“Well, maybe if you didn’t spend all day reading—” May prodded the book with its garishly-coloured cover with her foot. “Bizarre comic book porn…”

Why do atheists not love a G-d that does not stop punishing them harder and harder in this world and the next until they surrender to Him?

“Yep!” Claire chirped. “There’s this schoolboy, see, and he’s homeless, so he lives in this boarding house that used to be a hot springs bathhouse, which is cheap because it’s haunted, so he decides—”

Claire, one of May’s three flatmates, former university roommate, and best friend in all the world, shrugged expansively. “It’s a Saturday night. What else would I be doing?”

“So you didn’t meet any cute boys at the club tonight?” Claire called as she bustled about the small kitchen.

Why do men love to stink/being smelly?

“Perv.”

“I’m just a fan of your catch and release program.”

May yelped. “Hey! Your feet are cold!”

Here’s how we presented the character Claire when she was introduced, which the agent particularly singled out:

“Yuuna and the Haunted Hot Springs!” Claire turned the book around.

The agent had only one bad thing to say (the synopsis was crap; writing synopses is hard!), but praised the characterization and particularly how well we introduced a character’s personality quickly.

“You don’t need a cat. You can’t take care of a cat. You can’t take care of a ficus.” Claire flopped on the other side of the sofa and wriggled her feet beneath May.

“About wearing more clothes? How am I supposed to catch any fish if I don’t show off the bait?”

“Fine.” May collapsed into the warm spot Claire had just vacated.

“None of those either. Look upon the wasteland that is my sex life, and see that it is barren. Naught but a moggie followed me home.”

They both burst out laughing. “I’m right, though,” Claire went on.

“No way.”

“From the look of you, if you try to sleep now, you’ll spend the next three hours hanging onto your bed trying to stop the world spinning. Since you’re not going to sleep anyway, you might as well keep me company.”

Do that and you can ground your characters quite quickly.

“Why is that always your first suggestion? I do not need some tea. It’s three o’clock in the morning! If I have tea, I’ll never get to sleep.”

Essentially, what you do is show the character:

“I’m serious!” Claire said. “It’s staring straight at me.” She let the curtain fall. “Weird.”

“Exactly.”

Claire sat back down, legs tucked elegantly beneath her. “You are looking a bit sloppy,” she said, inspecting May through narrowed eyes.

In the kitchen, Claire set out a battered pair of mugs: May’s black, with “PEBKAC: Problem Exists Between Keyboard and Chair” in white letters; Claire’s white, with “This must be Thursday. I never could get the hang of Thursdays” in dark blue. She carried both mugs into the living room. “A moggie followed you home? Is this some weird Internet slang I’m not current on?”

“Claire, I—”

“Nope, I mean a cat followed me home. A black cat, to be exact. All the way from the club. Probably still out there, for all I know.”

“Thanks. You’re looking pretty ratty yourself. Have you been in that bathrobe all day?”

“Exactly.”

“Cute girls?”

“Number one, it’s not porn, it’s ecchi, and number two, why would I waste a perfectly good Saturday doing anything else?” Claire pulled at her tea and sighed. “The only thing that could make this day better is if you'd come home with some cute boy, so that after you kicked him out tomorrow I could live vicariously through you.”

Doing something they enjoy, that expresses their personality, and that is in some way unusual or noteworthy;