I was able to achieve typewriter text written on a wall, in front of which pedestrians were passing by, and the text was keyed out. Here’s how I did it:

Step 1: Footage. I shot on a tripod under consistent lighting, making sure to get a shot of the wall without any obstructions in front. On the same reel I had pedestrians walking by

Step 2: Freeze frame. I took a freeze frame of the empty wall and ran it as my lowest layer for the length of the clip.

Step 3: Duplicate. I duplicated the action clip, making three layers total. (freeze frame, and two identical action clips.)

Step 4: Difference Matte. I applied a difference matte to the top action layer, using the freeze frame as the reference layer for the matte. Tweak the settings here to make sure you get the right effect. Try to key out the background so you only have the moving figures. I also applied a feather at this stage to the top clip just to make things a bit more realistic.

Step 5: Text. Create your text layer in between the action layers, and set the blend mode to your choice. The result is the keyed out figures on the top layer passing in front of the text, which lies on top of the bottom action layer.

Good luck!