The short version below:
Day 1 -- Wouldn't It Be Cool If ...
Day 1 is the day we start batting around the ideas. We know where we are in the uber-plot, and we know where we have to start and finish. Now we need the idea that gets us from A to B, and Day 1 is where the greatest idea ever happens. And that's when one of us will spit out something like: "Wouldn't it be cool if ... " Excitement sweeps through the room. The blank dry erase boards no longer look so daunting. We've done it. Which leads us to...
Day 2 -- Um, There's A Problem
Everyone comes in early, excited to knock this sucker out, only to discover that after a night's sleep, maybe this great idea doesn't exactly write itself, that maybe it's got a few... issues. After a few hours of wrestling with the difficulty of our premise, we all decide to sleep on it again. Which leads us to...
Day 3 -- The Breakthrough
We all come back in, maybe a little later than the day before, and all with the same conclusion. The problem is unfixable. That is until the sun is setting and someone, thankfully, quietly pipes in with ... the breakthrough. At first it seems just a small thought, but in actuality it has huge repercussions. Riding the wave of euphoria, everyone heads home nervous. Is this a real breakthrough? Possibly. Which brings us to ...
Day 4 -- The Break Begins
As the discussion begins, suddenly ideas for scenes start popping up. Natural places for our uber-mythology to slot in appear. Connective tissue between episodes presents itself. All the ingredients seem to be there. And that's when we know we might have a ways to go but we know we have an episode.
Up until the end of the break that is, because that's when, once again, we stare at the blank board that taunts us. We can't possibly keep going.
Someone: "Wait. Wouldn't it be cool if ..."
And the room goes quiet.
Suddenly we're re-energized. We all realize it at the same time. It's Day 1 again and, just maybe, we can do another one of these.
'We'...that's the operative word. Most of us have to work it alone --- trying to grind through those 'stages' all by ourselves. But the room is mostly successful because one has an undying belief that the collective creative energy of the group will ultimately save or at least improve anything you might come up with on your own. If you can find a handful of colleagues you like and trust that you can exchange works in progress with, the feedback can be invaluable.
There's another major factor why it works this way on TV series: deadlines, and the fear associated with those deadlines.
You can and will experience the same daily 'stages' when working solo on your feature film or series spec sample...but without some sort of deadline hanging over your head and the fear it generates, it's wayyyy too easy to succumb to the 'problem' and give up on the idea after Day 2. Or, conversely, spend wayyyy too much time getting bogged down trying to make the solution discovered in Day 3 the best solution ever.
And thus you never really reach Day 4 and the actual 'break'.
When working on a series and its intense schedule of delivering scripts to be filmed, there isn't the time or luxury to either give up or wait for the perfect solution. You have to deliver.
It's next to impossible to reproduce that pressure if you aren't under contract and the deadline is self-imposed...but if you can somehow believe it exists, and work 'in fear' so to speak, you'll get a lot more accomplished a lot faster. And believe me, something finished and out there is always better than the almost perfect idea still in the desk drawer.