4.4. Stage 4
Once the superimposition was done, I returned my layers to 100% opacity. In the layer menu, I then created a layer mask on the first image, in the uppermost layer (Layer→Add Layer Mask→Reveal All). With the Airbrush at 100% opacity, I started to bring out the character underneath by brushing only on his area.
The technique of using layer masks is totally safe as long as the layers aren't merged or flattened, since you aren't destroying any of the pictures. You can bring out or hide any part of a layer you like.
As you can see, I had trouble with the center of the image. The edge of the layer mask gives you an idea of the struggle I waged. The problems were due to the original shot, objects being superimposed on each other, warped perspective, and visually distracting elements.
Thanks to the milling machine in the right part of the image, I was able to cheat in straightening the perspective. I only kept a small part of the machine: the horizontal table, which I tilted upward slightly and retouched with the Clone Stamp tool to strengthen the image. I then uncovered the rest of the panorama, including the picture of the last character.