A twist on animating a stroke is to use the new Scribble Effect.
Select a layer with a closed mask.
Select a Fill Type.
For varying stroke, increase Path Overlap Variation.
For more concentrated Scribble, decrease Spacing.
Scribble can be applied to more than one mask and will break up a mask it finds difficult to fill with one line. Check the Fill Paths Sequentially box if you are animating the start or end of a Scribble and want the masks to behave as one. If a layer doesn't have a mask, use Scribble in conjunction with Auto Trace, which generates masks. (Choose Layer>Auto Trace).
Use that Scribble for an alpha! Draw with white and set up the layer as a Luma Track Matte.