I think that in writing a story together with other people, negotiation and compromise are the key words. (Some see compromise as something bad, they give it a negative meaning. No, I don't mean that kind of compromise, but giving and taking mutually, and meeting the others half-way. And some people are really willing to do this, and I am thankful to them.)
Why do I say negotiation? Because the objectives of each character must be met. Not only mine and not only yours. And this is why, while I accept surprises and sidetracking from the initial plan, as our muses lead us, I don't like somebody (or me) throwing a major, life-altering event without prior discussing it on yahoo or in a pm with all the thread partners. Because it is not only their character's story, it is everybody's together, and everybody has to find something fun (or interesting) in it.
Usually, the answer is "yes". I had never said "no" to a plot ideas which made logic for the characters, however I have said enough times "yes, but let's twist it a little for this reason," so that all characters' goals are met and the story is not spoiled for any of the characters.
When entering a thread, I want to know an outline beforehand and find my own, original way through it. This way of working has several advantages:
1) for me, half of the fun is also outside the board, plotting with the others, on yahoo, our next moves... It is also a kind of spontaneity, but a different one - throwing ideas at each other (some thought before, some spontaneous indeed, which came at the result of the other's idea), analysing the potential reaction of our characters in different situations and what makes logic and what doesn't really - this way we are playing more scenarios before choosing the best one to write it.
We end in actually writing one idea, but we are playing with 3-4-5 alternative ones before deciding the one to be written, so it gets more fun than playing only with one idea! And even if 1-2 of those ideas exchanged happen to be silly, we still have fun with them before agreeing that it wouldn't be feasible for the x reason. Or other times we merge 2 ideas, one of each of us, in a new one, more interesting/ intriguing! The ones who are against plotting might allow themselves this experience once, for a try, and see if they haven't fun with playing with several alternatives before choosing the best one.
2) we end knowing better both our characters and the others', from this discussing the plot before. And sometimes new characters or NPCs emerge from here.
3) Some ideas don;'t make sense and they are discarded, but on one's idea the others add too. And when it is a negotiation process so that everybody's goals get met, both the story and the character developement (each character's) are enforced, without running the danger of somebody spoiling the story. Because if 2 or 3 people strive to write a good thread, then it would be a pity that one of them attempts to ruin it only because of a miscoordination! But if talked in advance and properly analysed, his idea might have merits and be included in the story somehow (probably not in its entirety, but only bits and pieces or slightly changed) without ruining it.
4) it reinforces the friendship and community feeling between writing partners.
5) it helps with some details which go"building towards" a main event; I like doing it in respect to almost all my characters in a story. I mean nothing comes out of the blue, when it arrives to that point, even if it might be unexpected for the reader at that moment, he might say: "I could have known it before if I was paying attention to little hints".
I had tried the surprise road too (both "totally surprise" version and "agreed beforehand in a way, but partner changes mind and does the radical opposite"), but it definitely doesn't fit my style. My writing gets slowly building towards something, and once we get to the scene we have been building to, looking backwards, there are a lot of little hints to the readers (which could have been interpreted the right way or not , but they are crumbles on the road). If the castle is shattered and the opposite happens... all my work, all my writing, had been for nothing, and my character is looking like a fool... worse, I as the writer seem a fool in these conditions.
You would say that "it happens in real life too" for people not to get the expected outcome, and I am not in favour of "happily ever after" scenarios when it is not the case. I am in favour of a heads up given to the writer a while before, not being put in front of the accomplished deed, in order to find the best way for the character to deal with the situation. Yes, the character might still look like a fool if this is desired, but the writer doesn't anymore.