There are tell-tale signs throughout the story which hint at the 'surprise' which is to come.
While looking for 'The Bell and Dragon' to take a room there, Billy Weaver notices a sign " Bed and Breakfast" and somehow feels compelled to go up and ring the door. It is as if he has lost his own will and is under some kind of spell, with the house itself beckoning to him as if it were a person.
When he does so, as soon as he touches the bell the door swings open and the landlady is there, smiling and waiting for him. She even tells him his room is all ready just for him, as if he were some kind of privileged guest.
Then there are, of course, are the landlady's stuffed pets - the dog and the bird which Billy presumes at first as being alive. The fact that she would stuff her formerly loved pet is somehow bizarre, even morbid.
When led to his room upstairs, Billy realizes that it's been quite some time since the landlady has taken on boarders and that at present he's quite alone. Then the discussion turns to the names of the two boys in the registrar, which have a vague familiarity about them, as names which have appeared in the papers. Billy can't quite place where he has seen the names before, but they seem to be on the list of "missing persons."
The landlady then mentions the physical perfection of one of the former boarders and the fact that there is not one single blemish anywhere on his skin. (Why in the present tense, for that matter? And how, pray tell, would she know that, unless....)
Finally, when she offers her future victim some tea, it has a slightly bitter almond taste to it. This is the taste of arsenic, and by this point the reader can very well guess what is coming next....