Colleen Dewhurst who played the small role of the Countess (?) in Act III said in an interview at the time that it was so nice to take a curtain call and look out into the house and see teeth, as in smiles. She had played tragic roles of O'Neill et al, for so long, she forgot that a comedy would affect the audience differently.
This is a sweetheart of a play when it is done well.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher