What exactly can a player (hero) do when a card is played. Can the player narrarate how others are reacting to the circumstances or is that totally up to the GM?
For example...If the bad guy shoots an explosive rocket at the mayor (with a 7 of clubs) while the hero is too far away to actually stop the missile etc. can the player change suit in order to have the mayor jump out of the way in the knick of time, or is the Mayor now toast no matter what happens (short of the player CANCELLING the shot).

I love rules questions!

The short answer: The player can definitely ink (i.e. narrate) stuff for people other than just his hero, as long as it doesn't contradict the cards played, the as-yet-unresolved Stakes, and is acceptable to whomever has those characters as Aspects on their sheet.

I'm going to assume that "firing a rocket" is using the "attack with weapon" category of conflict (although I could see it as "using a power" if the power is "superscience gadgetry"). You play 7 clubs and say: "The Annhilator fires a missle directly at the mayor!" I have three options: (1) I can play the 7 of clubs and Cancel and ink something like: "No, you don't fire because my sidekick previously disconnected the wires in the missle" (negating action) or "No, you don't hit the mayor because he jumped out of the way" (negating effectiveness) (2) I can play a club ranked 8 or higher to Escalate and ink something like: "Yes, you fire the missile and I bat it out of the way of the mayor using this telephone pole" or (3) I can play two cards of another suit and Change Style, inking something like: "Yes, you fire the missile, but that doesn't matter, because I smash you with an Atomic Kung Fu Chop (or whatever)."

NOTE: The real fate of the mayor depends heavily on what the Stakes of the conflict were, and whether he is an Aspect on anybody's sheet. Remember that if the GM's Stakes are "hurt the mayor," then the mayor doesn't get more than cosmetic injury until the hero yields the entire page of conflict. No single card played can ever undermine or negate the Stakes of the scene. Also, if the mayor is someone's Aspect, then he can't just die unless he gets to Devastated (and possibly even until he hits Transformed, depending on how you ink it).

Clearer?

I'm starting to get it.  Lets say the mayor is not anyones aspect. Lets say whoever set the scene stated it takes place at the mayor's state of the city address. The Annihilator's plan is to inject the hero's public identity's girlfriend with a mind control drug.  The Stakes for the Scene are... If the Annihilator wins then the Hero's girlfriend will be injected with the mind-control drug and will be under the Annihilators command... If the hero wins he saves his girlfriend and knocks the Annihilator out who will be taken to prison. Now the GM lays down the 7 of clubs and says "The Annhilator fires a missle directly at the mayor!"  He shoots the missile because he figures the hero is nearby and knows that will keep the hero busy while he infects his girlfriend.
Can the player control the mayor in anyway other than to cancel the 7 of clubs?
Who controls the crowd when the missile is shot (runs in panic, drops to the ground etc.)?
Who controls the police?

Hi, sourflower (BTW, what's your real name?)

Okay, if the mayor & the crowd aren't anybody's Aspects, and they're not part of the Stakes, and they're not part of the Plan, then they're window-dressing, just like manhole covers and trees. Anyone can describe the mayor or the crowd doing stuff that explains or enhances the card they just played. They obviously aren't that important to the players, which means they're not that important to the story or the reader. Couple things to keep in mind, though:

(1) Don't think about this in a wargame-like "who's playing piece is it?" kind of way. Instead when I say "describe comic book panels" I really mean "describe comic book panels." Think visually. Maybe the missle streaks off-panel and we don't find out where it hit or what happened until the fight is done. Maybe I grab the Thought Balloon, hold it over my head and say "Good thing the police chief tackled the mayor out of the way of that rocket. Now, to deal with The Annhilator!"

(2) Remember that the entire group can offer pencilling suggestions for what happens to the missile and the mayor. If someone you said last panel that the mayor cowered and started to run away, and on my panel, I want him to stare down the missile bravely, then we might be snapping the reader's suspension of disbelief. If someone at the table thinks this is too inconsistent, they need to mention it at pencilling. Anyone can suggest other reasons the mayor could stand up rather than run. Talk it out if there's a big disagreement. Which is a nice segue into...

(3) If folks at the table care so much about the mayor to argue about him, maybe he should be part of the Stakes, or an Aspect. Keep an eye out for this. If one player takes a shine to an NPC, the least you can do is note it on their Scratch Pad to be considered for an Aspect for the next Story Arc.

Thank you! That clears soooo much up!