Here's a description of Joan's character from the play.
SYLVIA GLENN - Attractive, flashy, former movie star, now around sixty. Still lots of pep and sas. Ambitious to reactivate her career, many times married, but now broke. Beneath the rather shiny veneer, there is a warm woman who shows herself when she feels secure. In films she usually played the strong, pushy type, like Crawford. She hardly ever played "a lady of breeding" She and Aretha have a special relationship which allows them to put each other "on" without any hurt feelings.
And Linda Evan's character ....
LEATRICE MONSEE - (Pronounced Mon-say) - Leatrice is around the same age, give or take a few years, as Sylvia. She is a far different kind of movie star, one who always played nuns, nurses and saints. Her public figure was always one of "goodness" but beneath the surface, she was just as strong as Sylvia. A Loretta Young or Joan Fontaine type with a reputation for sweetness but underneath lurked what came to be known as "an iron butterfly"