A killer will disguise himself or herself as a different character, only to appear as themselves in the next scene. Impostors are handled well by Agatha. They are successful at hiding their true persona or maintaining a false identity for years only to cover up a murder from the past (or to set up revenge and murder in the near future). People with false identities exist because: 1) an item was "planted" on the "wrong" person; 2) two people swapped identities with one another; 3) it enables the villain to achieve a grand goal, such as inheritance; 4) of course! the true person has already been murdered
Suspect everyone. No one really should be above suspicion. Just possibly in Agatha's stories, one may encounter any of these scenarios: The detective may be the murderer. The sidekick or associate of the hero may be the murderer. The policeman investigating the murder may be the murderer himself. The narrator of the story may be the killer. The client soliciting help from the police may be the killer. A person of any age may be the murderer. An intended victim may be the killer. All the characters/suspects may have conspired in the same murder. The most well-respected character in the book may be the murderer. The hero of the story may be the killer. Or, the character that we love the most--believe it or not--may be the killer.
Everyone lies. Yes, they do. Even the detective or hero in the story. It is an axiom of human nature that we lie. Characters of Agatha's tell fibs for various reasons. Plenty of people other than the murderer have guilty secrets of their own. Their dishonesty, theft, or unfaithfulness may have nothing to do with the murder investigated. Of course, killers are terrible liars. They may remove suspicion by trying to take away their own life (faking it by maybe drugging or poisoning themselves). Very clever.
pay attention to what Agatha actually puts into words. She has a great command of language. When it comes to language, don't think that sometimes she wasn't paying attention, getting too old, or that the printers made a few typos. Pronouns may seem mistakenly placed or ill-used. Not so! Don't assume that people's names are just for one gender only. You'll be in trouble. Pay attention to what people say to others: 1) in half-heard conversations or 2) in scraps of paper likes letters or wills. Think of the over-all context that this information was found in. I always say about books: if it's in the book, there's a reason the author included it there!
Don't assume that the "important" clues are the same ones the detective say are "important". That's the author--Mrs. Christie--misleading you, confusing you, or simply giving you a headache for you to say to yourself "Ah, that's not that important."
This ties in with the art of "red herrings". According to the Random House Unabridged Dictionary, a source found at Dictionary.com, a red herring is "something intended to divert attention from the real problem or matter at hand" or "a misleading clue". So, the important points or clues of a detective contain some that are not that useful to you as a reader. The detective (through the author) states that some points or clues are vital, when really these are leading you to a wrong solution or wrong killer. You become preoccupied thinking about minor details taking you away from possibly a correct solution.
If a suspect doesn't have a motive, then surely he/she can't be the murderer, right? Not exactly. If the suspect wasn't around to possibly commit the crime, then surely he/she can't be the perpetrator, right? Not exactly. There is such a rule of motive versus opportunity. "Suspect everyone" is a universal rule in solving detective stories. Sure, that's "easy". But, the reader can't help it if there is no motive. There is always a motive in a Christie story. It might not be noticeable; Christie has made discovering a motive difficult in some of her stories. She did that by design. Still, suspect that person. What about any opportunity of performing the crime? Still, suspect that person if it seems impossible for the suspect to have committed a crime. Perhaps the villain had an accomplice, or there was a prepared trap beforehand. Still, suspect that person.
Christie had a box full of motives to pick from. Her books have motives such as inheritance/wealth/gain, revenge, blackmail, theft, independence, self-righteousness, jealousy, concealment, meddling (knowledge, eyewitness, interference), espionage, love/hate, gossip, fear, and (yes!) even rehearsal. If the motive fits, but the opportunity didn't present itself, then dig deeper. As mentioned before, maybe the villain prepared the crime ahead of time, used an accomplice, used a disguise of some sort, or simply used "smoke and mirrors". About that last part: all is not what it seems to be in any crime scene. Don't forget the murderer (and the author) is putting on a "show"--there are many different angles (perhaps a "behind the stage" angle, too) that you musn't forget about. There's some sleight of hand here, and it's up to you to work out the magic trick.
That is the "opportunity" rule to remember in solving a Christie mystery. If the guilty party wasn't (apparently) there, there must have been another means of committing the crime. We touched upon that just right now with using a partner, using a disguise, or by means of trickery. In a detective story, there must be opportunity. It's not easy to see sometimes. Christie does have her sleuth or hero explain how the crime was committed to satisfy the reader. If she didn't do this, she'd be doing the reader a disservice. Because of this, opportunity can be worked out. You know it's so-and-so because of his/her motive? Having trouble on the opportunity? Try looking at the details leading up to the crime.
Look even for "minor" details that lead up to the crime. If a suspect notices something and mentions it, but never again--be suspicious. Be also alert if the author provides a description of the scene of the crime--before and after descriptions. You may have to re-read that portion of the story to answer: who was there, who wasn't there, how did the rooom look like, or what event took place before the "discovery" that distracted everyone momentarily. Christie liked to complicate the story with the deaths themselves. A death doesn't necessarily mean "murder". It could've been suicide, suicide dressed up to appear as murder, or even an accident. If you can't answer the motive and/or the opportunity factor, take a look at the death and ask yourself: was it simply an accident? Did the victim commit suicide? Sometimes these are included in novels with multiple deaths, simply a way for Agatha Christie to confuse you further.