I Fast Cast a smiley face at you!!!
And I deftly parry with my own fast cast scowly face! 
Chicago eh? Where? I'm in the Burbs of Chicago myself.
I should start a Chitown Tribunal!
I am in the actual city on the far north side, but I am originally from Lombard (& other portions of DuPage).
My esteemed sodale, Lady Phoenix, is also in Chicago. She is still fuming about the amazingly clever and efficient way I delt with mundane "authorities" who dared to accost a Hermetic magus 
Wow, Lombard is right by me. I'm in Villa Park.
Casting deadly spells on Mundanes is such a guilty pleasure for the PCs.
And remember, a magus was once charged with slaying some 100 mundanes (knights, soldiers, squires, camp followers) and was easily acquitted by simply pointing out that there were no surviving witnesses to tell the tale.
Well familiar with Villa Park
Unicorn Comics!
Well, I have been playing "good guy" Flambeau for so long, it felt good to cut loose with a ruthless and remorseless Apromor magus 
If they didn't try to arrest me, no one needed to have gotten hurt 
Magus Trentus of Flambeau! Yes, indeed a contraversal and interesting figure, to say the least.
I'm leaning more and more on house ruling that Fast Cast is not an attack spell option.
Here's my reasoning: (We'll compare a Fast Magus and a Fast Caster Magus)
- Fast Casting will negate initiative. Initiative (for a magus) involves Quickness, the Fast Cast Merit, and possibly Mastery Abilities like Quick Cast. Fast Casting though requires Quickness, Finesse and you can add the Quick Casting Mastery Abilities (to either Initiative or Fast Casting). The Fast Cast will outstrip the high Initiative opponent when taken to moderate or extreme levels. Fast Casting can gain the benefits of a high Finesse which can be achieved quickly with merits of Affinity or Puissant Ability (or both) plus the Quick Casting Mastery. The only way the high Initiative magus can improve his Initiative is to keep raising his Mastery of the Spell. The Fast Caster can raise two different Abilities to improve his speed though. Technically both can try raising their Quickness, but the Fast Caster has the advantage in this contest. There's no real incentive or care to roll or achieve a high initiative when you get a second chance to zap them first if you lose initiative. In fact you would really not want to get initiative here, because then you lose your chance to fast cast in response to the other magus, sort of.
Because Ars doesn't provide a lot of round by round game mechanics and Actions (Like D&D and the new Warhammer rules) there's no mechanic really for a magus that won the initiative to hold his action and respond. He either acts (and risks being out fast casted by his opponent) or does nothing so as not to trigger a response. Now then both might start trying to fast cast in response to the other fast casting them, but the winner of initiative is the one that's actually at a bit of a disadvantage here.
By keeping fast cast as a defensive response spell only that prevents this for the most part. Initiative still stays relevant, and the fast caster becomes the defensive wizard that still wants to win initiative so he can try his own offensive spells. The battle becomes more of a chess game, and speed is still important as is initiative.
Another option that prevents this is to make Finesse a part of the wizard's initiative when casting spells. Now the Fast Wizard and the Fast Caster are on even footing (the fast caster actually has a slight advantage in that he can take the Fast Caster merit which doesn't help Fast Casting spells, but that's fine IMO, you paid for a merit to get that advantage).
Anyway, starting to ramble.
Oh yeah, I usually go the extra mile to Graham Crackers these days though. And Games Plus is my gaming store, have you been there?
I am constantly at Games plus, mostly friday nights but this brings up the question:
Mundane says to Mage, "Come with us, we will take you the seneschal for questioning?" Can a mage fast cast as response (the senario I presented above (Fast casting two spells: wizard leap and fast cast/multicast wound that weeps.). He is reacting to the event of a you are under arrest declaration.
Can a mage fast cast in response to any sort of action even if not defense. He looked at me funny so I fast cast?
Can you cast more than one spell in reaction to one event or does each fast cast have to be in response to a different action/provocation?
These are the questions that make fast cast confusing. That the fast cast can be offensive rather than defensive doesn't bother me. I fast cast a fast cast mastered pilum of fire to burn the attacker to a crisp before he can attack me is valid.
They burned me on a transaction about ten years ago. I used to be big into the comic shop business.
Games Plus is cool. I used to go there all the time for my Ars material. Too out of the way nowadays, so I just order online.
qcipher: I see your point and I think it is very relevant.
What I want to say, is that one way or the another, there is problems with this system. You pick your problem.
If you differenciate between offense and deffense, would you allow a spell that turns back an arrow onto the archer as a defense? Breaking the weapon used ? breaking the arm? the person ?
My real opinion is that the combat rules of Ars Magica are flawed.
A very eloquent exemple, is that while there is initiative modifiers depending on what action you are doing, you declare your action after initiative is rolled.
Don't get me started on that subject, I could bring a lot of problem in those rules.
The reason for those flaws is that there is no action rules in Ars Magica. That's right: there is Combat rules, but no action rules. Here is a hint of that on page 171:
It's not "People with highest initiative make their action"!
The system works well as long as you mostly have warriors that already know which weapon they are going to use. Warriors vs warriors, with mages not doing inventive or complicated things.
Actions rules for Ars Magica 5th have yet to be written.
About one fast cast and one normal spell per round...
Same here. I was of the opposite opinion before. At first because Erik Tyrell made me re-think about it, and I've seen the wisdom in that, because it causes less problems (IMHO) with the system.
However, check this out: (from HoH: Societates, p.21) It's Dimicatio, a duel of two mages competing, each trying to counter the opposite spell caster, and the winner is the first to get a spell through (not countered).
As far as I understand, and if I was wrong I apologize, you can Fast Cast in response to anything, as long as you are reacting instead of proacting. Than means I can use any thing as a stumulus (and as I recall, I thought he was tring to lay hands on me to arrest me), but the moment I was placed under arrest I reacted by Wizard Leaping. The gurad that stabbed my grog with the spear, it is clear he had murderous intent because that must have already been what he was planning. He cannot "fast cast" a spear thrust to my leaping. He had to have been doing that already, otherwise he would have needed to wait until the next round.
You can continue fast casting as a reaction to that same event or to any other events of that round, so long as you keep succeeding your fast cast roll. Oce you fail you are at an end. You can still cast that last spell, but it may not matter any more at that point.
Did this suddenly turn into a Saga Locator?
...if anyone is around UCLA I'd be keen... also do online gaming on OpenRPG...
Ahem, back on topic.
Per the normal rules fast-casting is basically an Abort action. It seems so terribly arbitrary, though.
After all, it's the same process as normal spellcasting, yes?
You're basically trying to shove a spell out as fast as you can.
What confuses me is why you need to wait for something to happen. What, in character, prevents your magus from trying to rapidly cast spells in response to nothing in particular?
Yep, I agree with you.
The worst is this: you are better waiting for another action, because that other initiative will be lower, making the fast-cast roll easier...
I agree, this is where the rules make no sense - in character - as they are.
We need new rules ! 
I suggest you consider coming up to Grand Tribunal California later this year. I'm sure it will be worth it for the gaming alone, and you'll meet lots of other Mythic Californians who you might be able to start up a saga with.
Some of the biggest problems with it is the nebulous descirptions of what can be done in a combat round as well as what constitutes a reaction to an action.
For example, could you react with a fast cast spell to just simply losing initiative? Basically you lose the initiative and so open fire? That's why I have a problem with the offensive spell being Fast Cast, what exactly are you reacting to? Aren't you in fact just pulling the trigger before the other person who just beat you in initiative?
Breaking it down just a bit eliminates these problems, declaring actions for example. Warhammer and D&D are geared to small scale, model using, swashbuckling type battles on grids and maps. Ars Magica combat seems to be an overall melee with such things as spacing, timing, actions being an abstract.
Some options to try and elminate this would be to maybe make it similar to Certamen. The offense casts their spell, the defense tries to defend against it, then they switch. The reason that's important is then the winner of the initiative is otherwise at a disadvantage; once you cast a spell you can't fast cast. So you pop a spell on your high initiative and hope for the best, but the other magus has choices. He can try to tough it out or resist it and then cast a spell, or fast cast a spell or even multiple spells. The first wizard though is out of choices, he can only resist what the other magus does with no chance to fast cast anything.
I'm not sure what would be best though.
<Serf's Parma>
The RAW is quoted by Iudicium, as being: "Thus, a magus cannot cast more than one normal spell, or a fast-cast and a normal spell, in the same round."
I don't have a book here, so let's assume this is the exact spelling in the book. There is a large difference compared to the quote by Erik Tyrell. Erik removes part of this sentence, a part which is vital for a correct reading.
The rule stipulates how many spells a magus can cast.
A magus cannot cast more than either u a normal spell[/u] or u a fast-cast spell and a normal spell[/u] in the same round.
The fast casting rules further explain that you can cast even more spells when you beat your opponents initiative, taking a -6 penalty on your fast-cast rolls.
Summarising: normally you only cast one spells per round. But as a reaction to sudden events, you can also fast-cast reactions to that sudden event.
IMO there is a big difference between:
"A magus cannot cast a fast cast spell and a normal spell in a round."
"A magus cannot cast more than a normal spell, or a fast cast spell and a normal spell, in a round."
Or a similar example of different sentences:
"A grog is not allowed to drink wine in a day."
"A grog is not allowed to drink more than a bottle of beer, or wine, in a day."
That quote means that if you cast a normal spell, you are done. A normal spell takes a full combat round. There's no time to do anything else. IMHO, the reading is "You cannot cast more than one normal spell NOR cast a fast cast and a normal spell, but you can cast more than one fast cast if you are good enough."
You definitely cannot fast cast a multicast spell. Pg 83: "A fast cast spell is always cast with firm voice and bold gestures and the maga may not exploit any other spell casting options, as there is not enough time." Multicast is definitely an "other spell casting option."
The main question in the discussion about whether you can fast cast without something to react to is "what's the point?" It only takes a couple seconds to cast a formulaic or spontaneous spell, so if you are already free to act first there's little reason to take the -10 to casting total and +2 botch dice. If you aren't free to act first, then you are clearly reacting to something.
Fast casting is there for when the character is trying to ensure that his or her spell gets off before some event. If there is nothing to "beat", just cast your spells normally. You can certainly cast an offensive spell by fast casting if you find a reason to do so.
The wizard with the fast casting virtues is more likely to have the initiative and thus not need to take the -10 and extra botch dice, so I don't really think there is a balance issue with fast-cast spells negating initiative. That's what they are intended to do. If the wizard has invested the xp in Spell Mastery and Finesse and has a casting total that will survive the -10 penalty, then let them go to town as it were.
There's nothing wrong with Marko Leaping and then Wound that Weeps-ing the guard if he could beat the guard's initiative with his Fast cast speed at -0 and -6, respectively. He just shouldn't have been able to multicast The Wound that Weeps.