I've always wondered what percentage of plays are improvised..
Do most QB's stick to the book during playcalls or are most called plays broken up and improvised?
Any former CFL QB's in the forums? 
I would say that most if not all of the play is not improvised. Only time improvisation really happens is if there is a scrambling QB. All other times receivers should be running routes based on the play called and the defence given (as in if its man run this route, if it's zone run this route, etc.)
Depends. If you are playing against BCs front 7 and they get after you, then maybe a third of your plays probably get broken in the backfield. If you are playing the Riders, then maybe only 1 in 8 or 1 in 10.
it really varies. Henry, early in his career, only ran 10-12 plays on O, so they got broken down by the D a bit and he made thing happen. The only play that is really improvised consistently around the league is hot routes based on D lineup, which teams do not do enough of. The Als are one of the better teams at that.
I would have to agree that only a pressured QB really makes improvised plays, and I would say it is maybe 15% of passes.
Of course we are talking O, because there is more on the D…the Riders of 2010 ran almost exclusively improvised D.
some times it happens a lot.
depends on who is running the offense and what you have for receivers.
when Calgary had Flutie and they also have receivers like Pitts Sapunjis and Crawford.. a lot of their plays were not set as far as patterns go.
Pitts' roll a lot of the time was to go and find an opening and Flutie would throw to him.
there's no such pattern as that. Pitts just goes to a location that he looks for and waits.
it depends on the QB and the defense they're facing. you don't want to have to improvise. it's nice to be able to, but it doesn't matter how good you are at it, the ideal result is that you get to execute the play as it's drawn up.
so if you get a bad snap, you're going to have to improvise. if your receivers don't get open and the pressure gets there, you might have to start improvising.
but in general, you don't improvise very often. it's dangerous if you have a QB that's good at it because you get people running around and forgetting assignments on defense.. but obviously, you don't want to do it if you can help it. so it's usually pretty rare, unless the defense is having one heck of a day or your centre doesn't forgets how to snap a ball or something.
and receivers don't improvise. they have option routes where the read the defense and adjust their pattern to take them to an opening, but the possibilities on option routes are still part of the play. the QB can't just wait for someone to get open and then throw it to them or all they'll ever throw is picks. it's timing. and the QB HAS to know where his receivers are going to be (or at least, are supposed to be).
I really like this explanation, as well as a few of the other ones given on this thread, as well. Having been a quarterback, I would like to add just a little bit.
The receivers, running backs and quarterback all read the defense presnap.(Number of men in the box, position/depth of corners, position of safety, and how they respond to motion, etc.) If there are lots of men in the box (Linebackers crowding the line), then the backs will be looking for blocking assignments, and the receivers and qb will be thinking of their hot routes. If there are few men in the box, then the primary pass routes will likely be run. There will also be clues from how the defensive players shift to account for motion in the backfield. Pass routes will adjust differently if the offensive players read a zone or a man defense. This is all designed into the play, and is not improvised. Think of the various routes as contingency plans.
Once the ball is snapped, all hell pretty much breaks loose, but in a planned way. If the men in the box come forward, the backs pick up blocking assignments, and receivers run quicker routes. Likely, if there is big pressure, one receiver (likely the slot back) will automatically work to the middle on a slant pattern, trying to find the hole left by the blitzing linebacker or safety. On a corner blitz, the safety will rotate out to the side, also opening up the middle. Keep in mind that these hot reads are not improvised, but rather are read number 1 that everyone has to make.
If the men in the box drop out, the offense has to read that, too. Dropping linebackers provide better coverage, so the running back(s) will abandon their blocking and find a predetermined spot in the flat or shallow route. The receivers go out on their assigned routes, reading zone or man coverage and choosing the predetermined option. This is why it is so critical that the qb and his receivers be "on the same page". If a receiver reads zone, and a qb reads man-over, then the throw will be going to a different spot than the receiver, and the likely result is an interception. Again, this is not improvised, it is preplanned. Teams practice this, a lot. It is important to note here that the qb is NOT waiting to see what his receiver is going to do. The ball usually has to be released BEFORE the receiver even makes his break, or else the coverage can close on the ball.
Once on the route, the receiver will then read what his cover is specifically doing. Is he taking the inside, to force the throw wide? Or is he protecting the outside, and relying on help from the safety? This will also adjust the route, and the qb will have to read this, as well. Again, these options are preplanned, and not improvised.
All the while, the qb is also reading the defense to see which receiver is getting open. Every play has a primary receiver, then a secondary, a third read and then the outlet to the running back. All of these reads are done as a progression...if A happens, then execute this, if B happens, then execute that...and so on. This applies to everyone on the offense (including linemen and their blocking schemes)
In fact, even when a play breaks, and they do improvise, that improvisation is also sort of pre-planned. When receivers see their qb flushed and scrambling, they have specific assignments. Deep receivers will usually break shallow, and short receivers will usually break over the top. They will definitely have a "panic zone" to go to, so that they don't all end up in one place.
The only time a TRUE improvisation happens is on a pick/fumble/tipped ball/blocked kick, etc. Even on bad snaps, the receivers will likely play it as a blitz, and go to their hot routes.
Its actually more complicated than I described, but you get the gist.
Depop is really good at describing the Xs and Os...he did a great breakdown on another thread some time ago on the differences between Man-Over, various types of Zone, and Man coverage, and how to read them. If he comments, the insights will be good.
Oh, and I also wanted to add...
Running plays are almost never improvised. They happen too fast.
The ball is handed off at a predetermined spot that is a planned number of steps behind the line of scrimmage. The linemen are assigned blocking in such a way as to open up a hole at a predetermined spot, and the back runs at that spot. The hole is there, or it isn't. If it isn't, then the run is blown up for a loss or little gain. Occasionally, a back will improvise and "bounce", but coaches generally don't like it when that happens. Linemen get crossed up when the back goes rogue, and a lot of twisted knees tend to happen on such plays.
I'm not saying you tell a world-class starting back not to find a different hole, but if you watch carefully, I think you will see that backs bouncing to different holes and making a big run of it is more the exception than the rule.
Of course, if there is a fumble or a bad exchange, all bets are off.
It depends alot on the QB the O.C. and the talent they have to work with, The trend in the CFL latley seems to be very little improvisation, O.C's are calling the plays now as well,( meaning that if there is a problem in execution the Q.B. is supposed to just protect the ball ,as in nfl) Some QB's who can scramble and call their own plays are allmost unstoppable I.E. Doug Flutie - IMHO
Hit-em-hard's explanation is excellent. To that post I'll just add a few points:
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Protection schemes are what get audibled the most by the QB at the line. If the QB reads blitz and the protection isn't what he wants, he'll adjust, either by motioning a receiver or back to the line to help with protection (chip-block or real block), or by changing the whole formation, which doesn't happen very often.
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Teams frequently employ what's known as the stutter-count to help them read defenses pre-snap, adjust, and then run the play. On a stutter count, the receivers hit the LOS as if the play is about to happen but stop at the last moment, so that the offense can see whether the D is in man or zone or combo, who's covering who, who's playing man press, who's giving a cushion, who's blitzing, etc. The QB can then audible a protection and/or go to his hot read post-snap.
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It's important to note that the reduced time between snaps in the CFL doesn't allow for the kind of extensive audibling you see from someone like Peyton Manning in the NFL. There just isn't enough time. Calvillo is probably the QB who audibles the most in the CFL, and even he usually only has time for a quick protection audible (e.g. slide-right, or motion the fullback into a tight-end position).