- Alex: Were there people at the cinema?
Me: No, because it had just opened.
Grammar: Fine, Alex is talking about a past event with simple past and your comment is about something that occurred prior to that.
Let's put that into a declarative form to make it easier to grasp:
There were no people at the cinema because it had just opened.
- Alex: What had you cooked for dinner?
Me: I’d cooked some..
Grammar: For Alex to say "What had you cooked for dinner?", there would have to have been something implied in the conversation that precedes his using that tense or something that actually occurred prior to it.
- Alex: What had you cooked for dinner? [so that they all got sick.]
Me: I’d cooked some chicken ....
So, 2) would not be right unless there is a precipitating event, real or implied.
Declarative: I had cooked chicken for dinner before they got sick.
The past perfect always implies a prior event or refers to an actual prior event where simple past or past continuous is used.
- What had you done so they became so angry?"
- What had you done when they were swimming in the pool?