Trait Assimilation

It’s well known that characters change during a story. They’re going somewhere, conquering worlds, falling in love. Often a novelist forgets that people rub off on each other. It’s human nature. Sometimes, people pick up speech patterns, sometimes they try new things. The hero and heroine educate each other. They introduce each other to their worlds. They become one. Like an old married couple or identical twins, they finish each other’s sentences. The possibilities are endless. In that, lies some of what is fun in writing.
This might seem trivial or difficult to show in words, but that’s what the reader expects. They want to see characters as real as you can write them.

The last scene in Some Like It Hot, 1959 (Tony Curtis, Jack Lemmon, Marilyn Monroe)

https://youtu.be/qWS2NVX6VP0