After Effects How to reveal text as a runner moves past the shot

Hi there,

I've done this before but have completely forgotten how to do it!

Basically, I have a stationary shot of a runner running past the camera. I want to reveal text as the runner moves, using a mask. So the text stays still but the further the runner goes, the more text is visible.

Hope I've given enough info, I know there's a relatively easy way to do this but I just can't think.

Thanks in advance
Not sure why you say Relatively easy if the runner is moving then it's going to require lots of layers and letters on different layers letters on different layers merged together yet separate Sounds like tedious work
Okay, well could you shed any insight as to how it's done then..? I have done it before and all that I did then was create an invisible mask that tracks the runner and reveals the writing as it moves, surely this only requires two layers?
I envisioned the runner running through the frame with the letters following that would be plenty of layers