Oh crud, if I tried to watch my grammar, "shows not tells" and everything like that in my first draft, I'd never get it done. Sure, I'll do a rough outline before I start, just to get a general idea of what direction the story
could go in but that changes as I go along and if I worry about those editorial details I won't follow the characters where they need to go (not always where they
want to go, but just enough so I can keep a lid on the tale).
So far, in my current WIP, the following surprises have occurred (much to my delight):
The original MC protagonist turned out to be the antagonist for someone that I
thought was only a buddy-type character (and this former buddy character is now the MC protag).
One character that I thought existed, doesn't-- and never has (danged antagonist is one heck of a liar)
Another minor character turned out to be more important then I thought he was and instead of being dead from the start, he's alive throughout the story.
A secondary, yet important character ended up surprising the snot out of me when she died in a freak incident (it really can't be called an accident).
Something that I thought would be a "big reveal" turned out to be just part of the story world.
And the biggest surprise of all? The new MC dies about 1/3 of the way through the tale. Yes, he's still the MC and yes, it works for this story.
In sum, I feel you should have a general idea of where the story is going at any given time (right now my current "ending" is something along the lines of "final confrontation between the antag and protag that crosses both realms and helps Fieldstone (another secondary character) give a reasonable answer to the Ramirez family." Knowing me though? That'll prolly all go to spit once I get there, but hey, that's the way I like it.