Maggie Smith was in "Mrs. Klein", a drama about a psychologist. I went to see it whilst in London.
This was 33 years ago.
That night the understudy went on.
ps It's a darn good piece.
Being a 2 person show, it can be more economical to produce, so, if ever you see it running somewhere and are in the mood for a thoughtful drama, I suggest you go.