There's really no right answer. If there were a rule, it would be to use the lowest frame rate possible. This means fewer total frames, which in turn means a slightly smaller file. More importantly, it means that low-end computers are likely to play back the animation at the intended rate. In addition, it can mean less animating as there are fewer frames to animate. The only downside is that you have to be very creative to create animations that use only a few frames to communicate an idea or a movement.
My monitor's refresh rate is 75Hz, meaning it blinks 75 times per second. However, I can crank the Flash movie's frame rate all the way to 120 fps. What would be the value of doing that?
Nothing, really. First of all, you'll likely find that if you set the frame rate to 120 fps, Flash won't keep up. If you're playing only a frame or two, it can actually go much faster than 36 fps. But even if Flash could display 120 fps (on a fast computer), it won't play that fast for the vast majority of the audience. Generally, going much over 36 fps is simply asking for trouble.
It was really helpful deconstructing that keyframing.fla file. Can you add some more files like that?
Sure. In fact, at the site (www.phillipkerman.com/teachyourself/sourcefiles) you'll find several other files that appear later in the book.
I've set my document properties to 24 fps, but the display on the bottom of the Timeline changed to 18.2 fps. How do I change it back?
First of all, this is a sign that your movie can't play at 24 fps. The 18.2 indicates that the last time you played the movie, it could reach only that frame rate. The frame rate set in the document properties has not changed. If you really want to change the display number, you need to access the document properties again (by double-clicking the 18.2) and click OK. Keep in mind that just because you set the frame rate to 24 fps doesn't mean Flash will play that fast. It will try, but it might not succeed.