This is an interestingly sensitive subject for me, considering at the last tournament I went to, Don't Ask, Don't Blink - I actually talked about a friend during pools matches... and didn't realize he was playing right in front of me. He lost the match, and didn't make it out of pools as a result.
I suppose a general rule is to simply shut your mouth if you are observing. Kels would tell you I'm pretty friendly during matches, talking and laughing, playing around, etc. His Fox is so annoying for me...