One of the most difficult parts of writing a mystery – and hence my usual starting point – is finding a motive for the murder. The problem is that there are a limited number of reasons why one person will kill another: money; hatred, anger, jealousy; to protect someone else; by mistake – and that’s about it.
This is definitely limiting. If you write a book about a very rich man who’s been murdered, for example, the probability is that it’s one of the people inheriting his wealth who did it, and the only question is which one. So, unless you intend to write a very flat book, you need to add other elements, or perhaps a twist. You might, for example, employ an ingenious method of killing the victim, which only becomes clear towards the end of the book. You might give all the suspects rock-solid alibis, so the detective’s job is to break what will turn out to be a very elaborate fake. Or it could be that he wasn’t killed for the money at all – or that he actually committed suicide, but made it seem like murder because he wanted someone to get the blame for killing him.
Thinking back to the thirty-odd murder mysteries I’ve written, I find that the motive is the thing I try hardest to keep from the reader, so that while he or she may have a sneaking suspicion who did it (and in any book, there are only a certain number of central characters who could have done it, because you simply can’t make someone who only appears briefly, on page 47, the murderer), he or she has no idea why they’ve done it.
The other thing about motive is that it has to be plausible (within the context of the genre) and it has to be there right from the start of the book, so that it is possible for the reader to work it out. You can’t announce at the end of the book that victim was killed because he had an argument with someone in Singapore thirty years earlier, if your reader doesn’t even know he’s been to Singapore, and if that argument hasn’t had any effect on the victim’s life after he left Singapore.
All-in-all, writing a mystery is a rather tricky business.