There is a lot to work through on a TV show or film. The concept has to be strong, your characters all solid and in place, the rules of the world locked and the format of the show nailed. These are fundamentals. You do not proceed into production until you have these things all fixed and agreed.
But with production looming and deadlines fast approaching, sometimes you just have to get moving. So out of these fundamentals, which ones can you afford to work out as you go along if things get tight? Which one can you let slip and work out in the episodes themselves?
The answer across the board here is: None of them! Work out what your show is!
Seriously, these things are absolutely essential to the health of your show. Yes, there are details that will get fleshed out as the series builds, some elements might change as you realise that there are strengths you hadn’t previously identified. But you have to know exactly what your show is when you go into production. You must have the format locked, the characters all worked out and fleshed out, the relationships clear, the world shaped and consistent.
Without those, you’re basically playing out your development process on screen. That is not a good idea.
Some things you can’t afford to figure out later. Decide on the fundamentals before you start.