I keep discovering new things about the universe I'm writing in. Often I'll go back and insert hints and make changes, but sometimes I have to leave those details in my head, ready to unveil at an opportune moment.
One time in my mid-teen years I overheard my father complimenting me. He didn't know I was close by enough to overhear him. What he said astonished me. At the time I thought of my writing as a game, a diversion, something fun that was for my amusement alone. Every so often I'd have an excuse to write something creative for school, and I had a lot of fun with that. I had no idea that my father had read any of it, though I often deposited my papers on the kitchen table in order to show my parents how I was doing academically. What he said to his friends that evening was that my stories were like a flower with lots of petals unfolding.
Boy, I sure would like to live up to that compliment. Anyway, I think what he saw back then was an effect of my writing process, where I race forward and then go back when I get a fresh take on something so that everything is consistent and carefully (I hope) foreshadowed.
I spent much of last night, late into the night, doing just that with a particularly cruel truth that Mark, the pov character, will discover far too late to prevent a tragic confrontation.
I love writing. It's fun. I think the only difference now is that I do think about how I can best help a reader in on that fun. It requires a lot more work, since the reader can't know what's in my head. But that's fun too. I can keep secrets, and then pounce with them.