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If there's a role you're playing and there's a great deal of material to explore because the person was real then it's a completely different preparation time and message to playing someone fictional.

"Emma Thompson Talks SAVING MR. BANKS, Her Preparation Process, Getting Input from Richard Sherman, Terrence Malick’s VOYAGE OF TIME, More". Interview with Steve 'Frosty' Weintraub, collider.com. December 20, 2013.
If there's a role you're playing and there's a great deal of material to explore because the person was real then it's a completely different preparation time and message to playing someone fictional.