Unpublished writers commonly ask 'how do I stop a publisher ripping off my idea if I send in an outline/story?' Of course, the answer is 'why would they?' Having ideas is the easy bit - writing the book is work. I have very, very occasionally heard of a publisher stealing an idea. On the whole, it's pointless (as well as unprofessional). They would still need to get someone to write the book, so if they like the idea, why not go with the person who is keen to write it? Unless they can't write for toffee, of course. But if you can't write for toffee, you're never going to get a publishing contract, so don't worry about it.
But that's rather a digression. What this is really about is how much you should say about a work in progress. Do you keep it a well-guarded secret in case people steal your idea? Or do you gossip about it endlessly? Most people who follow me on twitter (as @annerooney) know that I am writing a series of six vampire novels at the moment. Not many people know who the publisher is. I have leaked various plot details, but the very original premise of the series is not generally known. Writing a vampire series is hardly original. In fact, it's so unoriginal I feel I have to apologise for it constantly. But there are some things about this series that are very original, and those are the bits I don't leak - just in case.
I had to clear with the publisher that I would be talking about the series on twitter in advance of publication - in fact, while writing it. There's a sort of non-disclosure clause in the contract. I said I was going to talk about it on twitter and he didn't argue. The publisher is a laid-back sweetie and occasionally joins in the discussion on Facebook and twitter about the progress of #thosevampires (they have their own hashtag). But I would not make the central premise public at this point as that is risking not only my investment in the idea but the publisher's investment - and that's not fair. Plenty of people know that book 2 involves a nasty scene in Paris with a guillotine. But that's hardly enough to recreate the story - I'm sure everyone who reads that can come up with a completely different vampire story that involves a guillotine.
On the other hand, I'm currently playing around with three possible non-fiction proposals, trying to decide which to concentrate on - and I wouldn't discuss any of those on twitter until the contract's in the bag and the book quite well progressed. Whereas every fiction writer will do something completely different with the same details, the n-f books are entirely definable from the central idea. And although I like to think that my blend of experience and research skills is unique and no-one else would come up with quite the same book, I'm sure that given the basic idea many other writers could produce a very similar book. So about those potential books I can say nothing. Once I pick one, I can leak odd snippets of information and no-one will be able to guess the thread that links them. But until then, my lips are sealed.
I suppose the point is that it's fine to give away details of your work in progress as long as you don't give away enough of the central idea to risk compromising its originality. Maybe this is the grown-up version of 'will they steal my idea?' As soon as the publisher produces the catalogue, the central premise is there for all to see anyway. But by then, the books are in press and it's a bit late for someone else to start from scratch.
Small confession: I don't really know much about vampire novels, so my series is a bit out on a limb (but I didn't know that until I'd nearly finished book 2, so we're stuck with its bizarreness now). There - ignorance about how to write a vampire novel is the secret. Any help?