Hmm. I’ll have to make a note of that question for a future article because I don’t think I can give a good answer in a short space. One tip — you want to start with characters who are complex enough to excite you & also be able to carry multiple books. Don’t go for likable. Go for intelligent and resourceful but they’ve got to have some serious flaw. Would the 50 Shades male main character have carried several books if he wasn’t 50 shades of effed-up? Nope. This writer has some good advice about that in his medium story: https://medium.com/writing-for-your-life/why-save-the-cat-is-bad-screenwriting-advice-d157a415af61