'Would you like a cup of tea?' Jessica asked Maisie.
'Yes, please.' Maisie sat down in the armchair as Jessica went towards the kitchen door.
Jessica paused in the door way. 'Milk and sugar?'
'Just milk. Thanks,' she added as Jessica went into the kitchen.
Jessica and Maisie are just talking. There's no communication going on between them at all, and no communication with the reader. Yes, you could argue that Maisie has communicated her tea preferences clearly, but that's about it.
In real life, when we're talking, we're communicating all the time. Even when we're apparently not talking about anything of particular interest we're busy communicating. A discussion about books might be about intellectual point scoring or discovering someone who thinks the same way you do. An exchange about a celebrity wedding might be angling for information on how someone feels about marriage or body image.
We might not be aware of it, but in real life we usually have a hidden agenda when we talk to other people. An exchange about the weather at the bus stop may be looking for confirmation we're all in it together, tut-tutting over hoodies is dividing people into Us and Them.
So, characters have to communicate with each other, but more to the point, they have to communicate with us the reader. Often it's not in what the characters actually say, it's about how they say it, and what they think about it.
Maisie Watkins! Here, in her house! Jessica put her hands behind her back to stop them shaking. 'Would you like a cup of tea?' she asked, using her politest voice, the one that made Peter roll his eyes.
Maisie gave a brief smile. 'Yes, please.' Jessica clocked Maisie eyeing the sofa, before gingerly settling down in the armchair, her arms keeping well away from the sides. Damn, if only she'd have known Maisie was going to turn up she'd have done something about Tiddles - sprayed him with anti-moult spray or banished him from the house for ever or something.
Still, no time to think about that. Jessica paused in the door way, with what she hoped was a gracious smile on her face. 'Milk and sugar?'
'Just milk. Thanks,' Maisie added. Such charming manners, Jessica thought as she headed for the kitchen, praying that the milk in the fridge hadn't gone off.
Same dialogue, but this time it's communicating to the reader a lot of information about Jessica, Maisie, and the whole situation.