I think one of the biggest surprises I had when I started writing was that writing fiction doesn't start with an idea, followed by an opening paragraph, then a middle, then an ending.
When I was little and adults asked what I 'wanted to be' well - actually I was a teenager - this is a very teenage response - I'd say something like 'I don't want to be one thing, I want to do loads of things' and I'd list a few of them, and one was always 'write a novel'.
I imagined getting a blank book and filling it. And, although I'm not writing a novel now, I think that deep rooted idea of what writing would be like has stayed with me and reared its head at times when I've thought I'm not doing it right.
I've been prompted to write this post from a discussion I'm having with a writer friend about writer stuff, and in the conversation I've found myself saying 'I'm not a linear writer'. I know I'm not the only one, but it felt like a useful moment as I considered that.