1. "Light drew a rectangular shape on the wall. Suddenly, a shadow was/got cast on the wall. Then they heard the sound of a door opening/being opened."
2. "They entered the building and found themselves in/into a huge lobby."
3. "She goes over to a desk behind which a man is sitting/is seated." What is the exact difference? I mean is there a different meaning or is it just the same thing? Very little difference at all.
4. "He strokes her hair" Okay - but you need a period.
5. "There are all those/these things I have to do." Either - "these" can be a rather "non-descriptive, but "those" sounds like we've discussed what they are already.
6. When simply describing something: "The police officer, seated, seems nervous." Seems okay - a bit literary.
7. "Her car was parked along a curb." Okay, but "next to the curb" sounds more natural to me.
8. In the context of a store, what word would best describe the surface on which the store's name is written: "a marquee, a canopy, a billboard, a sign"? The marquee is okay but not a very commonl used word. A canopy hangs over the entrance -it's probably made of canvas or nylon. A billboard is a huge sign, like the ones on the highway. A sign is a multi-purpose word and fits your needs here just fine.
9. "The door opens. He sees the audience wainting for him. He takes a deep breath, then gets goes in."
10. "He is sitting in the backseat, on the passenger side, looking through/from the passenger window." If you are outside of the car, he can be looking from the winow, but not if you are inside with him. Through works either way. In both cases, he is looking at something in paricular. The more common collaction is "looking out the window." You don't need to repeate "passenger" before window, since he's sitting on that side.