Smith drily summarized her later roles as “a gallery of grotesques,” including Professor McGonagall. Asked why she took the role, she quipped: “Harry Potter is my pension.”
Musta been hell writing scripts for her. She looked like she was ad libbing it as much as following the script lines. She looked like somebody who would slap you in the face for simply ignoring her.
She's no different than Deniro or the like - she played herself in the various parts that she "acted." She was a willing accomplice in the various iterations of Project Mockingbird.
https://www.pbs.org/newshour/arts/maggie-smith-star-of-stage-film-and-downton-abbey-dies-at-89