The "Amy Tan Vow of Silence"
When my friend Kate T. and I were in college and just beginning to write, we told ourselves we would subscribe to the "Amy Tan Vow of Silence." Kate had come across an interview in which Ms. Tan stated she never discussed anything she was working on until it was finished.
When one is just starting to write seriously, I think this attitude is crucial. It is not the time to be getting advice from your friends or family whether you've asked for it or not, and it's not the time to be hearing people say "I don't know, that sounds kind of silly. Why don't you write about X instead?" The only opinions you should be paying any attention to are those of your characters.
At some point you will meet a kindred spirit, hopefully more than one, whom you can trust to be both encouraging and tactfully and constructively critical. I found that I could unzip my lips once I'd met a few people who fell into that category in my M.A. program. My friend Ailbhe's comments and questions about the plot of Mary Modern proved really useful, and her enthusiasm got me even more excited about what I was working on. And of course, if I hadn't initially run the premise by Kelly I might never have written it in the first place (I was afraid it might be too weird or ridiculous to develop any further, but she told me to go for it). It was the same deal this time around: I had an idea I was excited about, but I also had a few reservations, but when I told Ailbhe about it she encouraged me to put aside my hang-ups and get on with writing it.
But in the very beginning I think it's better to button your lip. Until the thing has taken a definite shape in your head and you've gotten a good chunk of pages down, and until you've found a true-blue writing partner, I think a vow of silence is the way to go. Tactfully decline to respond to any questions you just don't feel like answering, and don't worry about offending people.
At this point, I have no qualms about telling my close friends what I'm working on, but it's when people I meet in passing ask me about the plot of my new novel, what it's called, and how I'm categorizing it, that I tend to freeze up. All I tell them is that it's literary fiction-slash-fantasy, like Mary Modern without the faux-science. I don't even tell people what it's called. For some reason I'd rather wait until I know when it's going to be published to circulate the title. Another consideration is that if I haven't yet told my mother, my agent, or my editor what it's called, I'm sure as heck not going to tell someone I barely know. Having said this, I'm certainly happy when people care enough to ask.