I'm hoping to turn it into a mystery book series ... getting it on TV is only a dream, but of course it could happen. First I have to get the opening book published!
I don't know if it's true what Christie said or not, but I can see how it's possible. Make a bunch of characters suspects, decide at the end, and then go back and seed little clues along the way. Several mystery writers have told me they work that way; I myself knew the main suspect going in, but after finishing the first draft I did go back and fix up the clues a little.
I do have a "grand conclusion" at the end, because my male lead loves that kind of stuff and sets it up. (His wife rolls her eyes at him a lot.) However, I'm twisting that trope of the real murderer being there, just to keep people guessing.
Well, of course you could write a story and at the end decide how it would make sense. And then go back to write it again, so it does make sense. I'm just thinking it would be more economical to make a concept of what happens and who is the murderer and that, right from the start.
In Christie's stories (and those alike), there are no suspects - aside of knowing that someone must have done it - until the detective reveals what happened. That is the trick in it all, I guess. You (the reader) know it must be one of the persons who are in the story - but from whats happening you can't see how anybody could have done it, or why. Usually they even all have rock solid alibis. But at the end you see that the explaination was in the story right from the beginning. And just about everything that happened has got some meaning towards the conclusion. A perfect example is "Evil Under the Sun": you see everything as it happens right at the beginning, and still don't know what it means.
So when writing you either can make a plan how this is going to pan out before you start - or you keep changing things around until it fits. Which may be quiet time consuming to do, without making mistakes.
Thinking back on this subject, I am surprised how many books and series really have used the Christie pattern. And I'm not even sure she was the first - don't Conan-Doyle's Sherlock Holmes stories work in a similar way? Although they are more working with scientific evidence that the reader does not see at first.
But other franchises are pretty much exactly using the Christie style. The series "Murder She Wrote" for example. Did you know that it was one of the most successful series in US TV? Despite having this somewhat oldfashioned look.
And even up to this day this Christie method is popular. The Brits like it in particular.
A more modern version is "Death in Paradise", a series about a English police inspector who gets posted to some little caribean island. That is pretty much exactly like a Hercule Poirot story - only with beautiful girls, beaches and palm trees and so on. At the end they usually all gather at a swimming pool, or a terrace of a tropical restaurant, to hear who did it. Also the murderer of course. :) I mean, he could have long left the island to be on the safe side - but then it would be no fun, right?
In Germany we don't have series like that. Here it's always based on a few suspects, who are pretty much obvious from the beginning. And one of them is the murderer at the end. Not very innovative, really.
As I understand the story, Christie made it so several characters could be the murderer, then decided at the end which of them it would be. So, since all were a possibility, she didn't really have to do a whole lot of rewriting. Honestly I haven't read that many of her books, and can't say if the story is true, or just true in some cases, or not true at all. The way I see it, the best mysteries are the ones in which the reader can't see how any of the suspects fits, but when the criminal is revealed they slap themselves on the forehead. "Oh--it's so obvious now!"
But Conan-Doyle didn't work that way. It wasn't uncommon in the Holmes stories for the reader to have absolutely no clue what was going on, until Sherlock revealed details the reader hadn't been told about. Personally, I prefer to feed my readers enough information that they at least have a chance to guess who the bad guy is.
The important thing is to offer enough suspects, and enough twists, that you don't make it too easy for mystery lovers!
Murder She Wrote is still popular over here, and I watch it from time to time. One of the cable stations runs it most of the night--and even though it looks older, the stories still hold up.