A quick little tutorial from Felt Tips who shows how to create a writing on effect in After Effects, taking a more manual approach using the clone stamp tool to achieve a more stop-motion feel to the animation.

This shows how to use painting scratchback to create a write-on effect from a scan

There is also a great little tip for controlling the speed at which the final animation plays back by simply using the time remapping feature in After Effects, allowing you to control the playback speed easily.