First Person (Peripheral Narrator); John Steinbeck
Usually we here at Shmoop are against assuming that the narrator is the author. But East of Eden is a bit of an exception, because Steinbeck straight up tells you that he is a proud member of the illustrious Hamilton clan. He makes a point of including anecdotes about his mother, Olive, one of Samuel Hamilton's daughters and wife to one Ernest Steinbeck. Yep, our narrator is the author. Just look at how he shamelessly breaks out the first person during one of his tangents about his childhood:
Mary and I turned around and walked stiffly across the street and into our front yard. We felt horrible. I still do when I think of it. (46.1.19)
But you're probably thinking, "It's all well and good that Mr. Steinbeck wants to tell me the story of his family. But does that mean that Cathy and the Trasks are real too? Or does it mean that everything that he says about his family is fiction?" Good question, dear reader. To which we say this: this novel is undoubtedly a piece of fiction.
In fact, it's an allegorical piece of fiction too, which is to say that beyond being just a story, it's also using all of its characters as symbols (in this case, the symbols are from Genesis: Adam and Eve, Cain and Abel, etc.), which means that almost all of their actions and characteristics have symbolic precedents (Charles and Cal are farmers like Cain, for instance). The novel is self-referential about itself as an allegory too, like when Lee straight up discusses the significance of Cain and Abel during the naming of the twins.
But back to the narrative technique. So Steinbeck throws in a few characters based on real people here and there, and a few anecdotes that probably really happened (like his mother Olive taking a scary plane ride). Steinbeck is still telling the story from the point of view of an omniscient narrator, which means that he has to get into all of the characters' heads. That requires creative license, a.k.a. making stuff up
And Steinbeck has no problem whatsoever with doing that. Look at how he gets into his Grandpa Samuel's head:
As he rode through the light and dark of tree-shade and open his mind moved on. When had the Welshrats started crawling in his chest? He found it then—and it was Cathy, pretty, tiny, delicate Cathy. But what about her? (16.1.4)
In this section, we know that it's Samuel thinking these thoughts, even though they are in the third-person.
And since Steinbeck can get into every character's head whenever he wants, he has no problem shifting perspectives in-between paragraphs so that we get all angles of the story. It sounds confusing, but we register the change without even thinking about it. Look at how he does it to contrast Kate's and Joe's perspectives of the whole Ethel situation:
She felt that she could trust Joe, because she had in her files a notation relating to one Joseph Venuta who has walked away from a San Quentin road gang in the fourth year of a five-year sentence for robbery […]
Joe brought the breakfast tray every morning […] He knew that she was depending on him more and more. And Joe was very slowly and quietly exploring the possibility of taking over entirely. (45.2.9-10)
So Kate thinks that she's got the upper hand because she has some dirt on Joe, and Joe thinks that he's got the upper hand because he knows that Kate needs him to do her dirty work, but neither knows that the other knows what they know. And we apprehend all of this through a simple shift in point of view. Pretty neat, huh?