More about the "boiling" don't necessarily only make the character into a two frame clip, you want to make it look like a natural loop.
I personally enjoy making it into a five or four frame loop, An easy way to do this in flash is to make a second keyframe then highlight the entire object, from there in the tool panel should be the option to smooth which is an arrow pointing to an S. Depending on how far/close you are to the object the smoothing will be a tad different try to smooth the object one or two times. If it's too much command Z back to how the original drawing.
For multiple different lines do this by copying the original keyframe over and re-smoothing. The smoothness is different each time.
This way you are able to not have to draw/color the character/object multiple times and still get the desired "boiling" and or moving hold effect, and for non living objects I suggest just leaving them static.
i hope this stuff was useful to you, and if not I was still glad to offer a little bit of help.