I've recently come to the conclusion that my fantasy novel would be way too long to actually put everything into one book and still make it interesting. I'm starting to gravitate toward making it into a trilogy. However, this is new to me. I'm still in the planning stages mostly of my story: working on the plot, detailing the main characters a bit more, and writing chapter outlines. I'm starting to get a pretty good picture of what the first book will entail; the beginning, the ending (thanks to some great brainstorming suggestions on this forum), the character arcs and themes. Now my question, for those who have some experience with writing trilogies already or are working on one, is mostly: do you already make sure you have basically a whole outline in place as well for your second and third book before you actually start writing your first book? If not; how do you make sure everything falls into place? How do you make sure that you make the right 'promises' in your first book for instance, if you don't know yet what exactly will be the ending and outline of your second book? I'm currently watching the lectures by Brandon Sanderson at byu and greatly enjoying those, but skipping through the subjects I'm not sure i can expect anything on trilogies or anything of the kind. Very open to hearing some thoughts on this subject and possibly other questions that others have been struggling with.