I love how much foreshadowing there is in this story, and despite all of it I only caught the twist when he woke up to the tweeting birds.
‘Murder—help! murder,’ repeated the girl. ‘Somebody has played a trick on you, Monsieur. Who could be murdered here?’
Unlikely as it seemed, the girl herself might be playing a trick on him.
he was very conscious of the quiet observation under which he was being kept, and it frightened him a little. Was his secret written plainly in his face for all to see? Did this man, by reason of his professional calling, know that there was something amiss in the hidden grey matter?
Jack shivered at the thought. Was it true? Was he really going mad? Was the whole thing a hallucination, or was it a gigantic hoax?
the older man might have been waiting for such an opening. It was clear that for some reason or other Jack interested him.
‘It is a wonderful idea,’ she exclaimed.
Her eyes were alight with enthusiasm. Jack did not feel nearly so enthusiastic—in fact, he was inwardly funking it badly, but nothing would have induced him to admit the fact before Felise. The doctor acted as though his suggestion were the most natural one in the world.
Christie justifies everything really well: before we get to twist we already know that the girl just moved into the house, that our protagonist is a bit of an idiot, and that he's not thinking things through because there's a pretty girl involved.
I think the reason it sneaks up on you so well is that the motive doesn't appear until so late in the story, so when that slots all the other hints into place, you aren't really thinking about them anymore.