Another rich topic on today's American Theatre Facebook page: What's the best thing to say to a friend whose show you didn't like? My favorite response so far is probably Sasha Anawalt's "You did it!" There are also a lot of variations on my own preferred feedback for a show I didn't like, which is, "I enjoyed myself" or "I had a good time" (I just omit the rest of the sentence, which would be, "I had a good time thinking about what to make for dinner tomorrow"). I used this recently, when I interviewed Catherine Zeta-Jones after catching what I considered her so-so performance in A Little Night Music, and I added another not-the-whole-truth compliment: "You were lovely in the show." I mean, she is lovely, and she's in the show.
A lot of respondents recommend one fool-proof word I only learned recently. It was during post-show handshakes at a show I'd done the music for; I didn't pick up on it at first, but when a certain colleague of mine kept repeating this one word over and over again, and offering no elaboration, I got the gist. That one deadly word? "Congratulations."