If you move one point (vertex) on a mask it will create a new keyframe but the other points will not move. The motion between keyframe may appear to change but the individual vertices will still be in the same place. You'll want to start with keyframes only where the motion changes direction, then check the timing and make as few adjustments as possible. You will also always want to have the mask you are editing selected in the timeline so points are easier to select and manipulate. For masking the human form it is almost always preferable to use Rotobezier paths than Bezier paths.
Rotoscoping an entire human form with a single mask is not ever a good idea. That is an incredibly inefficient way to manually rotoscope. You want to isolate various parts of the subject, one mask for the torso, a mask for one leg, another mask for the other leg. In some cases, the most efficient technique for isolating a person in a scene from the background may require 10 or 20 masks. It sounds like more work, but it is by far easier. It is also often easier to temporarily motion stabilize a shot or use Mocha AE to do some planer tracking to help you with the masking.
You should spend some time looking at these two articles from my friend Scott Squires: Effects Corner: Rotoscoping - Part 1 and Effects Corner: Rotoscoping - Part 2. He is using a program called Commotion that is no longer available, but the technique is the same. Simplify the masking as much as you can by breaking it into pieces.
Here's a quick tutorial I did a long time ago showing how to set up and roto a very simple shot. Maybe this will help.
I should also point out that Rotoscoping can also usually be simplified if you use a combination of masking, procedural mattes, keying, and Rotobrush. If you show us a screenshot or point us to your video we can offer meaningful suggestions for removing the background in a more efficient and less time-consuming way. The last full body manual roto job I worked on required motion stabilizing, about 6 masks, rotobrush, and a procedural matte on three copies of the footage to be combined into a single pre-comp that could be used as a track matte to isolate separate the actor from the background. This shot required only four or five keyframes on a single shape layer right at the end of the shot to fix the part of the shot where the actor's shoulder passed in front of the replacement TV screen.
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