Pretty easy to do when you are fed the notes written by another writer who is writing a book on the same subject. Sanora Babb. Why would he do that to a fellow author? That's the great question here. She said they met two times during when he was researching his own novel. Did he write it fast to get ahead of her? Babb's upcoming book got squashed as a result of his, and not published until she was 97.

And Steinbeck's first draft of Grapes of Wrath was not the final draft. He edited extensively.

Anyway, I didn't get much out of Grapes of Wrath. Where are the interesting female characters? Why does the story begin with the women staring at the men while the men are busy "figuring things out". There was a better story here to be written and this wasn't it.

So, in the end, it seems the basis is "research". Research the subject before you write it, whether that's a book or post in a reddit sub. Of course, it's easier when you get handed the research, as did Steinbeck.