Obviously this is my perspective and your own experience may differ wildly, but I find I need different motivations to get seat in chair and write fiction vs. non-fiction.
First, by the end of next year, I’ll have 3 non-fiction titles in print to my one novel in print, but I don’t consider myself more successful as a non-fiction author than that as an author of fiction, just luckier I suppose. I had the right book at the right time for that market. (I do have two completed novels I’m shopping with agents and publishers – so hopefully this will change soon)
To write fiction, you have to be interested in the character, their journey their motivations and how they get past their challenges. It sometimes helps to know how this will all work out, sometimes it helps to discover this as you go. If you know too much, you can get bored in the writing, if you know too little, you can flail around for ideas on what does the character do next.
To write non-fiction you have to know what you are writing in advance. The work has to be clearly laid out and organized so that others can follow you. While in fiction you may wish to mislead your readers (such as in a mystery where you don’t want the reader to see the ending too clearly) and you can use unreliable narrators to help you achieve this, in non-fiction you have to be amazingly transparent and work hard to ensure that anyone who reads your work understands what is going on. Outlines are a must.
To write non-fiction, you must be exited about your subject matter. If you’re writing for adults, the books are often long. A book of 300 pages takes a long time to write, and if you are not exited about the subject, that will become an unwanted chore quickly.
I’m the kind of non-fiction author who wants to get my points down and then fill in the foot notes as I edit. I don’t recommend this, but it can help you focus on the production of text on the page. It helps me, but the review of the text looking for each place where I need to place in foot notes should be a separate edit than the read to ensure it makes sense, and read for typographical errors. This means you’ll want to leave yourself with enough time for at least three edits before you send it to your publisher.