Solving a Murder Mystery

Agatha Christie

I've been reading a lot of Agatha Christie lately. Nothing helps me unwind in the evenings like a good old murder mystery. After a certain point, I've come to learn a few things about her mysteries. Here are some of the things I've learned:

1. While the detective (whether Hercule Poirot or Miss Marple or someone else) will know who did it before everyone else because
1. They are privy to inside information
2. They are just plain smarter than everyone else.
2.  The detective won't divulge all the information they have until the very end. This is because
1. They are missing one pertinent fact such as motive and want to divulge everything at once
2. They want to wait until the perfect moment, when all the suspects are gathered together and the police are waiting in the other room to arrest the suspect at the perfect psychological moment.
3. If the detective decides that the murderer deserves mercy, the detective can let them know privately that they are discovered and allow them to take an easy way out, such as suicide.
3. When two people die towards the beginning of a book, one of the following is true: 
1. One of the deaths is a red herring, 
2. The first was a rehearsal for the second death, 
3. One of those deaths was either an accident or a death from natural causes, 
4. The second person to die was a witness to the first murder
5. The second person was an accomplice or suspect of the first murder
6. Or a combination of the above, or none of the above.
4. If someone disappears completely and cannot be found by the police, one of the following is probably true: 
1. They are dead, 
2. The detective is hiding him or her for some reason, 
3. That person never really existed i.e. they were the figment of someone's imagination or they were another character in disguise.
5. The murderer is always the last person that you suspect because of one of the following reasons:
1. The murderer is the narrator, and left pertinent details out of the narrative
2. The murderer is working with the detective to help solve the crime
3. The murderer was suspected very early on but for some reason was ruled out
4. The murderer was some minor character (local police, maid, secretary, etc.) who was in some position of trust.
6. In order to come up with the solution before the "reveal" a reader must pay attention to every detail:
1. Never take for granted anything that isn't stated by at least two people. Anyone can lie.
2. Beware of people who have gone abroad or disappeared or reappeared or don't have much history. They are often an impersonator, or have some history we don't know about.
3. Pay attention to people who are on vacation or otherwise out of the way when the murder occurred. Who can say they didn't come back unbeknownst to the other characters?
4. Watch for details. What is in the wastebasket? What was on the desk? Who tends to show up at just the right (or wrong) moment?
5. If the story gives times of comings and goings, make a note of who you know was where, when. 
6. Watch for discrepancies between what different people say, or what they say happened. 
7. Minor things are usually the biggest clues.
8. The murderer is almost never the obvious person. If all the clues seem to point at one person, realize they are being framed and look at who has a motive to get that person out of the way as well as to murder the person who died.
9. Watch for random possible connections. i.e. Someone has a relative they've never met abroad somewhere, Some other character lived in India or Peru for some time before coming to the scene of the crime.
10. Make note of descriptions of people. Similar characteristics between seemingly unconnected characters may be signs of unknown family members. 
11. Motives can include money, fear, jealousy, hate, family reasons (they want to marry someone else and their spouse wont give them a divorce) mercy, self defense, or revenge.

What would you add to this list?