We're working on an opening sequence. I proposed a couple of short scenes. Other folks came back with scenes that I felt were long and contained too much information.
As I wrote them, my general principals in approaching the scenes are:
1. What is the minimum context the player needs in order to understand what is going on? We need the opening sequence to tell us who the main character is, and what his relationship with the other key figures.
2. It is usually better to provoke the player to ask a question before we answer it. We need to provide a certain minimum amount of context (see above) so the player can ask a question that makes sense. Beyond that, it's better to provoke the question and then answer it later, than to give the player information they haven't asked for yet. This keeps the player engaged -- they pull themselves into the story rather than having story pushed at them.
So long as the player isn't actually confused, let the player wonder why things are happening.
If we provide more than the minimum context, we don't know which information is the important part. Instead of getting wrapped up in the characters, we're processing information. When we're processing information, we're not asking questions. When we're not asking questions, we're not getting engaged in the story. We're also not getting to the gameplay!
Only resolve a mystery once the resolution is more satisfying than the mystery.