Ahem, having relatively recently had some training in animation, here's some blithering about traditional animation.

8 fps = choppy animation but still reads as movement. Old eastern europe/USSR style animation was done with this amount since it's cheaper.
12 fps = Default disney style animation, kinda the workhorse of animated films. (as in, most stuff is done with 12 fps)
24 fps = For extra fast and smooth movement, you have to try around with this. Don't do an entire thing with 24fps if you want to complete it in a timely fashion.

Traditional animation runs at 24 images per second, so the different framerates here are more about how many -different- images are shown. So with 8 fps each frame is shown 3 times, with 12 it's two times (hence the term "shot on twos") and with 24 every frame is shown only once.


Not sure how useful this is for you, thinking back, but it might help with experimenting. :U!

  It seems you guys are confusing fps (frames per seconds) with keyframing.   Frame per seconds means exactly that, you have certain amount of frames per 1 second of film/animation.  You can have 24 frames in a second in 1's which means there is 1 drawing per frame, in 2's which means you have 1 drawing every 2 frames, 3's which means 1 drawing every 3 frames, etc.  There is no rule defined on what is considered right, there are standards which makes things easier for all of us but doing an animation in 6's is not a bad thing, it is just unusual and rarely applied by most animators.  Do not limit yourself with what Disney and other studios have done, they follow guidelines to reach their goals and they look good, sure, but that doesn't mean any other method should be blacklisted.   There are tons of animators out there who mix and match their actions with 1's and 2's even 3's and 6's to make their animations have more impact.  The only way for you to see this is to study the animations you like and figure out how exactly they did it.  That's like the best thing about animating..  Don't know how animator X did that awesome super kick look so awesome?  Stop the animation and study it frame by frame, it's right there, there is no secret juice or formula, there is just things that work on certain actions and things that don't.  Experiment and figure out how to mix and match, but whatever you do, don't limit yourself to what is printed in paper.

Now, when we talk about keyframing we are really talking about using a system. In order to lay down your action clearly it is easier to simply "break down" the movement into steps.  You start with the first most important movements, the ones that either lead the action or define a motion (such as an anticipation, a squash or a stretch, etc).  So for example, in a walk, you would key frame the drawings that define the walk itself such as taking the first step (when the character stretches the left leg to move, then the passing position (when the body is switching sides moving the right leg to the front and letting the left leg stay behind), then the landing position when the right leg hits the floor and the left leg is the one stretching to catch up with the body). I just did a 3 keyframe walk, you can chose to play this 3 keyframe walk in 24fps, 12fps or 4fps.  Most of the time if you play this animation at any fps you are going to have a crazy woobly loop of nonesense because you only have 3 drawings defining an action.   This is why for a normal walk at 24 fps least 9 keyframes per leg are recommended or "required".  If you draw all the 9 keyframes and then play it, your animation is not going to be woobly anymore, it will make sense.  Sure it won't be a perfect walk (because it is missing frames) but it will look like a walk.

 Since all you have right now for that walk are 9 keyframes you still need more information for the eye to read a smooth action.  This is when you start to lay down inbetweens,  which are the drawings between the keyframes that help the eye fill the blank spaces and complete the action.  You can even subdivide those inbetween into "major and minor breakdowns" which are sort like keyframes between keyframes.  So you have a Key frame in frame 1 and another Keyframe in frame 8.  This means that between your drawings there are 7 blank frames, so in order to continue filling out the animation you would then draw the Major breakdown, which would be the frame between KF 1 and KF8, in this example that frame will be the dead middle frame 4, but it can really be any frame depending on your timing.  Now you have 2 KF and 1 Major breakdown, so now you need to draw something between your KF's and your Major breakdown, this are called minor breakdowns.  So your drawing between KF 1 and MB 4 would be Minor breakdown at frame 2, or at frame 3 which ever works best for you.  You can follow this process with the rest of the animation allowing you to organize your planning in a more efficient way.  So now when you are planning your animation all you have to do is think in steps.  Start with the Keyframes, then move into Major Breakdowns and finally do the minor breakdowns, all the while checking your animation that it read wells and all.  This will help you keep track of your action, your spacing, your timing and your posing without wasting time going back and forth fixing single frames once you are done.  Key framing is a method of organizing your work and is there to help you create a clear and smooth animation it is by no means a law and in fact in 3d, all that is used is keyframes and inbetweens, there is no major and minor breakdowns, unless you assign them so.

  That's another wonderful thing about animation, that even inbetweens can affect the overall impact of the action.  Richard Williams explains this very well by telling you that when you create an action never inbetween in the perfect middle of two drawings, always try to aim for a side to lean on.  Meaning, if you have a key frame on frame 1 and another on frame 4 and your animation is on 2's then your inbetween drawing should be on frame 3.  This drawing can either favor more keyframe 1, or be close to the action in keyframe 4.  Depending on your choice, your final animation can have a completely different impact and this is can either make or break your work.  It is early in this stage of planning your animation that you start to use spacing to smooth out your actions.  In animation there is a rule of thumb I follow, the less space between drawings the slower the action, the more space between drawings the faster the action (slow in/slow out's).  Same thing can be said of frames, the more frames you have, the smoother (which usually translates to slow) action, the less frames the less smooth (faster) action.  I might be misusing the word smooth here a bit, but I am referring more to the timing of the action more than the readability.  How an action reads has more to do with posing and spacing than with the amount of frames it takes to deliver it.

  Disney animation is good but is not the only good type out there.  Don't dish anime just because it is not Disney quality, anime does an incredible job with vfx and fast actions, they are very good at delivering action with a limited amount of frames which can be useful to learn for future projects.  Sticking to one style or dismissing another just because it is not "appealing" to you is limiting yourself as an artist and as an animator for no reason.  You can have an opinion on what is best, but that does not mean you are right.  Study everything and anything that moves and you'll soon learn that the only truth is that your eye is seeing things moving, how fast or how slow is just an illusion.  Hell the whole point of animation is to create an illusion so go ahead and just play around.  Give it a go!  Sorry if I confused anyone, I just didn't have time to do nice little drawings as examples, maybe this weekend?  Maybe not?  I am terrible at posting things on forums.