I used to work in a theatre (lights and sound tech), and the most uncomfortable situation I knew was when I had to talk to the performers after a piece that I thought was rubbish. Trying to say something not rude to someone who has just spent two hours on stage in a small intimate black box theatre, often stuff they had written themselves, is really hard. I would squirm and look away and try to think of something, ANYTHING, positive to say about it. After a while I came up with a way of getting half out of it: I would make up (if necessary) some technical mistake on my part and talk about that.
-Hey, what did you think?
...
...
-uh... I really screwed up the lights in the third scene, I hope nobody noticed.
Then I some times didn't have to express my opinion.