From Ms. Whitelaw's obit in the Times (my emphasis):
It was at the National, in 1964, that she first encountered Beckett’s work; the play was “Play,” in which she was one of three figures encased in urns obsessively recounting their adulterous entanglement.
Beckett was so taken with her that over the next quarter century the two became collaborators. She accepted his artistic vision without always understanding its explicitly rendered ambiguities. They read his plays together, discussing not their meaning but the most minuscule elements of the text — the pauses and sighs and guttural sounds as well as the words, the inflections demanded by the language, and his need, as she said in interviews, to remove the acting from the performance. “Flat, emotion, no color,” he would often caution her, she said.
He wrote “Not I” for her, an effusive, stop-start, stream-of-consciousness monologue — a devilishly difficult exercise in gymnastic diction — in which a spotlight in an entirely dark theater focused only on her mouth, the frenzied movement of her red lips and tongue providing the play’s sole physical animation. She performed it at the Royal Court Theater in London and later taped it for television.
I had never heard of Ms. Whitelaw before, but I was vaguely aware of the play "Not I." (Understanding Beckett, however, is above my pay grade.) In the video above, Lisa Dwan describes how she plays the part.