I really enjoyed this piece by Catherine Nichols about a literary technique invented by 19th century female novelists that she calls adaptation.
Adaptation is a kaleidoscopic way of understanding human nature, and a novelistic technique for showing that character isn't fixed. In real life, people change constantly, depending on who's in the room, or what they've each understood of the others' nature and mood.
Here's an example from Pride and Prejudice:
The first time Mr. Darcy tries to express his interest in Elizabeth, he asks her to dance, and she refuses. Later, he sees her reading, and he comments to other people in the room that reading is important and his library is huge. Really great library at Darcy's house. Elizabeth, however, doesn't take the hint. Any shy person might recognize the arrows in his flirting quiver-standing around near her and saying to his friends that he likes the things that he thinks she likes. It's as effective for him as it usually is for the rest of us; she doesn't know, or doesn't want to, that flirting is taking place.
Then, the next time Mr. Darcy is alone with Elizabeth and his friends, he adapts. He makes an unflattering observation about Mr. Bingley's personality, offered to Elizabeth as a gift. He's changing his approach based on a comment she made in the previous scene. He can only change within the range of his own character, which is shy (he'd never say this in another context), clever (no one fully gets the insult except for Elizabeth), and sort of mean. It's an incredibly efficient scene, and it's how Darcy, a man with few lines and no third person narration spilling his secrets, can be as well-developed a character as Elizabeth herself.
For my money, P&P is one of best novels of all time. The adaptation technique goes a long way toward explaining why it's such an effective lens into human nature.