Fischl's Wikipedia entry is here, and here is his web site that contains many examples of his work.
Due to his need to portray people in those psychologically ambiguous situations I referred to, he needed capture-the-moment body poses and gestures difficult or impossible to obtain from live models. Beyond that was the need to get correct effects of light and shade on his subjects. So Fischl necessarily was drawn to the use of photography for reference material. This is what classical illustrators usually were doing by the 1940s.
Another consideration was composing scenes. Again he borrowed from illustration by creating overlays, one to a subject, and moving them around to establish the ensemble best fitted for artistic and story-telling purposes. Early in his career he made use of glassine to create finished works on that support material.
In recent years Fischl has been relying on digital photography, using Photoshop to manipulate the positions of subjects to achieve what he feels is a satisfactory compositions. He credits his wife, landscape artist April Gornik, for getting him using that software.
A 2012 exhibit at the San Jose (California) Museum of Art dealt with his use of photography. The museum's website page dealing with the exhibit is here.
Here are a few examples of Fischl's use of photography for his painting.
Krefeld Project. This seems to be pre-Photoshop.