I saw her on "Charlie Rose", and I was VERY impressed with her!! I agree that her character, is a reflection of herself.
As for:
"...Lena has a detachment Hannah can't achieve. Is this what is meant by having a "alter ego"?"
Well, I don't know if the "detachment" part, is an alter ego----but, I think "Hannah", in GENERAL, is the alter ego part.
As for the character getting ridiculed for putting herself in her work (something I'm just SURE happened to Lena, IRL), they always say: "Write wha'cha know", and I think she does that, VERY well.
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White female; age 59; diagnosed Aspie.
I use caps for emphasis----I'm NOT angry or shouting. I use caps like others use italics, underline, or bold.
"What we know is a drop; what we don't know, is an ocean." (Sir Isaac Newton)