Christie creates suspense in And Then There Were None in multiple ways. Suspense is a feeling of anxiety or uncertainty that occurs when people don't know how a situation (or book) is going to turn out or what is going to happen next.
In this mystery, a group of people is gathered together on an island by a host they do not know. That, to begin with, is mysterious and suspenseful. Why were all these people invited to this location or offered jobs here?
Second, the host doesn't show up, but he does leave a recording. The recording ratchets up the suspense or anxiety level even higher. On it, their host accuses each of the guests of committing a murder that they got away with.
To make things even more interesting, one of the guests, Anthony Marstan, dies almost immediately, as his drink was poisoned with cyanide. The recording had accused him of murdering two young children by driving recklessly with them in the car. Soon after his death, the remaining guests and the two servants find a broken "Ten Little Indians" figurine and realize that Marston's death corresponds to the "choking" death of the first of the Ten Little Indians. That this murder was well-planned also adds to the anxiety as it suggests the murderer is intelligent and thinking ahead.
By this time, the suspense level is very high. Who found out about all these people with unsavory pasts, and who brought them here? Who is murdering them? Who will be killed next? The number of unanswered questions multiplies the suspense, along with the fact that more people obviously are slated to be killed.
The suspense is compounded by the fact that high waves mean nobody can get on or off the island. The killer must be among them—but who could it be?
As more and more people are murdered, the anxiety builds.
Christie creates suspense, in sum, by depicting a situation in which a mysterious person brings a group of people to an island, traps them there, and slowly kills them off, one by one, as if they are sitting ducks. The pages keep turning as we wait to see who will die next and how it will correspond the to the "Ten Little Indians" rhyme. We also want to know who masterminded this elaborate plot.