For my multi-chapter stories (there are character based novels), I have always written short(ish) biography on all the main characters. This includes, what they look like, behave, evolve, catch phrases, and a little background story to support each character. This helps me to keep character constant, except for the deliberate changes that happen as characters evolve.
My story plots are all on a spreadsheet, which covers the over all story, and each chapter and what needs to happen. It there to keep me on tack, so I know exactly where I am heading and the route I should be taking.
Having said all that, I wouldn't say that I keep things on a tight leash. There is always wiggle room to change things, and I do, especially plot details. They always evolve as I write, though I try hard to keep plot changes true to each characters personality.
On short one chapter stories, I write a couple of lines, or use a photo to remember what each charater looks like. The rest is is in my head and I just get on and write it.