A new idea is delicate. It can be killed by a sneer or a yawn; it can be stabbed to death by a quip and worried to death by a frown on the right man's brow -- Charlie Brower
I rarely share a new idea for a novel or even a picture book, until I've written the first draft. The delight of nurturing a new story, the excitement of discovery, the burning lust for what comes next, is for me somehow quenched if I talk about it. For me, writing is a process of discovery. If I talk about it too much, I don't want to write about it. I lose interest.
Not everyone works this way. You need to know what works best for you. I jot down ideas as they come, walk and walk and walk to pound them out (oh, and walk and walk and walk as I write, too, during and in-between drafts) but I don't want the enthusiasm for my fledgling ideas, that to me seem so exciting, so desirous of pursuit, to be flattened by indifference, or crushed or overwhelmed by input from others, however well intended. It's only when I have a fair idea of what I want that idea to be, of how it will grow (of course, that's not to say it doesn't veer off in strange directions once I start to write) that I can even begin to tentatively share those ideas with others.