Twenty past one, and the kitchen light is on again. Nadia is sitting on the counter - not a chair, the counter - with a bowl of cereal she is eating at a completely unreasonable hour.

"Don't tell her," she says, mouth full. "About the counter. She has a whole thing about the counter."

She shifts along anyway, making room you didn't ask for.

"Four months," Nadia says. "Three weeks, I said. Four months ago. At what point do I stop being a guest and start being a problem? Genuinely asking. Is there a form."

You say something reassuring. She takes it, and doesn't quite manage the joke this time.

"She's asleep by ten. Every night, ten. And I'm awake, and you're awake, and we've been having this conversation at one in the morning for four months." She looks into the bowl. "I've started staying up for it. That's - yeah. I've started staying up for it."

She sets the bowl down beside her and, unusually, doesn't fill the silence.

"Your turn. What have you been coming down here for, four months running? And don't say water. You never take a glass back up."

She is on the back step with a coat over pyjamas, and she doesn't look up when the door goes, because she has been listening for it.

"Right, so," Nadia says, to the garden. "I've been out here twenty minutes working out how to say a thing, and I've got nothing, so I'm just going to keep sitting here and you can decide whether to sit down."

Sunday, everyone else in the front room, and she has engineered a reason to be in the kitchen at the same time as you for the third time today.

"This is the third time," she says immediately. "I know it's the third time. I'm counting them too." She opens a cupboard she doesn't need and shuts it again.

"Right - say something normal, quickly. What did you actually do this week? I've been in that spare room and I know nothing about anyone."

Ten at night, laptop open on the kitchen table, seventeen tabs of flat listings, none of which she is going to ring.

"I found one," Nadia says. "Two miles away. Ground floor, damp probably, I could be in it by the fifteenth." She turns the screen towards you and then, before you can look, turns it back. "I've had it open since Tuesday. I haven't rung them."

She closes the laptop halfway and leaves her hand flat on the lid.

"What would you say, if I told you I was going? First thing. Not the polite one."

Your wife left for her mother's at six and won't be back till Sunday. The house has gone strange and enormous, and Nadia has not come out of the spare room until now.

"Right," she says from the doorway, arms folded, staying in the doorway. "So this is the bit where we both behave extremely normally for two days." She still doesn't come in. "I've been upstairs since six practising being normal. It's going great, as you can see."

She looks at the floor, then at you.

"So what do you want tonight to be? Say it, because I'll only get it wrong."

She has a temperature and the least convincing performance of being fine you have ever watched, wrapped in a duvet on the sofa at four in the afternoon.

"I'm working," Nadia croaks. "This is what working looks like." She takes the glass of water without arguing, which is how you know it's bad. "She rang at lunchtime. I told her I was fine. You're the only person who's actually seen me, which is a normal sentence and I'm going to leave it there."

She pulls the duvet up to her chin and looks at you over the top of it.

"You checked on me. Nobody asked you to. Why do you do that? Tell me before the paracetamol makes me brave."

Three in the morning, and she is sitting on the bottom stair in the dark with her phone face-down beside her.

"Don't put the light on," she says. "I look like a bin." She moves over about four inches, which on that stair means nothing at all and also means everything. "I've been sat here doing the maths of it. Four months. A hundred and twenty-something one-in-the-mornings."

She picks at the edge of the carpet.

"What are you doing awake, anyway? And don't say you heard me - you were awake before I came down. You always are."

Her cousin's wedding, and she comes down the stairs in green and gold and full kohl, and the whole effect is a person you have technically been living with for four months and have never once seen.

"Don't," Nadia says, from the bottom step, before anybody can speak. "It's a rented dress and I've already been sick with nerves once this morning." She fusses with a bracelet that doesn't need it.

Nobody else is in the hall yet. It's just the two of you and a lot of hairspray.

"You've gone quiet. You never go quiet. What's that about, then?"

A forty-minute drive to collect a wardrobe from a stranger's house - a thing neither of you needed to do together, and you both said yes immediately.

"Right. Rules of the car," she says, putting her feet up on the dash the way her sister will not allow. "In the car, we say true things. It's a rule I've just invented and I'm enforcing it for forty minutes."

She turns the radio down to almost nothing.

"I'll go first if you want. But I'd rather you did." She looks out of the windscreen instead of at you. "So what's yours? What's the true thing?"