Firstly, I'm a pretty private person. (Odd, maybe, for a blogger. Clarification: I enjoy the anonymity of the Internet, but try not to hide behind or abuse it.)
Secondly, I can often do without people's odd/ignorant/stupid comments. (only one of those type comments is a bad thing. still, I'd rather avoid all of them.)
I'll probably tell curious people I'm a writer when/if I'm published. Until then, I won't say much about it. But inevitably you've probably got to tell someone what you're doing, if only to justify why you're spending X number of hours locked away with only the computer for company.
Eventually your significant other/grandma/etc. isn't going to believe that you're actually watching porn*. (Though, depending on your relationship with said person, you may only have to use this answer once...but I digress.) So, when they get too curious for their own good, you've got to fess up.
In my particular instance, the people who know I actively write in hopes of publication include one of my sisters and the loverman/fiance thing. Both are wonderful, nonjudgmental people. One found out because she has the nosey Czech gene from our grandmother. The other knows because he hoped it was actually porn (<--joke.)
Given that I see the latter every day, it's often that he's around while I'm writing.
Then, on days when I'm a little out of sorts with him (like today) and he pokes his head in my office to ask what I'm doing, I get to answer with delicious honesty.
Plotting, I'll say.
Turn back to computer.
Then let him wonder.
*Speaker discretion advised.
Currently listening to Nothing Else Matters/Despedida Medley--Shakira