My memory of Glenn Close's performance is actually similar to your description of Sigourney Weaver's. (I've never seen the movie, but I'm not in the least surprised you would say that. I thought her performance in "Vanya and Sonia and Masha and Spike" was embarrassing.)
I remember being so excited to see these three movie stars onstage in "Death and the Maiden" ( I was young), and I was disappointed. I thought they were very false throughout. There was one moment where Glenn Close's character had to run offstage to vomit, and it seemed like any freshman acting student would have done it with greater truth.