cakmpls and papersky have been talking about lying and telling the truth, and it's all very interesting, and I recommend that you go to their journals and read the recent posts and comments sections if you have any interest in these topics. I just wanted to reproduce part of what I said over at cakmpls's, slightly edited, after some discussion of McCarthyism:
[...] I think it's important to keep the phrases, "I'm afraid I don't feel like talking about that right now," "I'm afraid I don't think that's any of your business," "I don't share that sort of thing except with my closest friends and family," and "That's more complicated than I'd like to go into right now," all in ready circulation, with their attendant rephrasings. It's very important to combat the idea that anyone who doesn't feel like talking about every last little detail of their lives has already told you what those details are.
I got upset with someone for spreading the gossip that an old friend's fiancee was not sleeping with him, because I considered it exactly the same as spreading the gossip that she was: none of my business. And worse, it introduced the concept that those friends' sexual choices were legitimate topics of investigation for people with no direct knowledge of same. I can't think of very many things they could be doing together that I would disapprove of, so it's not that I want to protect them in case they have something to hide. It's that their relationship is not my relationship.
I also routinely refuse to answer that I don't know that someone is pregnant (before she's announced it), because if I made a practice of saying, "No, X isn't pregnant as far as I know," and then later I said, "Well, I can't tell you anything about that," it would be the same as saying yes. It has to start long before the specific question comes up if it's going to work -- the answer always has to be, "I'm sure if X had anything like that she wanted you to know, she would know where to find you." I can't think of a situation wherein I would consider it shameful for one of my friends to be pregnant, so again, it's not that I want to protect them in case they have something to hide. The idea that not all relationships are my relationships and not all stories are my stories is very important to me. So is the idea that the stories that are mine are mine not to tell as well as mine to tell.
(Does that mean I think you can't tell stories that start out, "I have a friend who..." or "I know this person who..."? No, of course not. But there's a difference between relating something that's a matter of public record and taking it upon oneself to share information someone else has made it clear they would prefer to keep private. And of course sometimes there are reasons to share things someone would prefer to keep hidden. But that doesn't mean that the default is that we all have to know all the details of each other's lives.)
(As for fiction...oh, that's sticky. When I wrote "Swimming Back from Hell by Moonlight," the basic scenario at the beginning was something major that had happened to a friend of mine, but with the sexes reversed: the main character's sweetheart has died in his sleep, unexpectedly and young. I wrote it deliberately for my friend Andrew, whose girlfriend Chris died in her sleep in her mid-20s. The story is dedicated to Andrew -- and also to leahbobet, but for different reasons -- and I was thinking of him when I wrote it. I was thinking of how he spent his days immediately after Chris's death, how he sounded on the phone, what he said about it. I don't think this counts as trying to tell Andrew's story, because there is significant divergence, and because it doesn't tell anyone who doesn't already know that the Andrew in the dedication was in such a situation. Andrew liked the story and was extremely touched that I'd written it. But there was the possibility that he wouldn't, and that it would offend him that I had even made the attempt -- and frankly, that possibility worried me. I sent him a copy of the story before I started to send it around to editors. But I'm really proud of it -- I think it's a damn good story, frankly -- and if he'd said, "Look here, Mris, you really screwed this up, and I'm hurt and upset, and I definitely don't want you to try to get it published," I don't know what I would have done. I am a cold enough fish that a part of my brain would have demanded, "What part did I get wrong, so I can fix it?" But it's possible that the very existence of the story would have struck him as wrong. The defense I have here is that I know Andrew well, and I know Andrew's taste in and reaction to fiction pretty well, too, and if I hadn't known him that well, I don't think I would have been moved to write a story under those circumstances. But I freely admit that this may be an attempt to cover my butt in the mantle of "it's different when I do it because I am smarter than people who screw it up," and that mantle may be, to mix my metaphors thoroughly but at least to stick with textiles, false colors.)