How does the author create suspense in "A Sound of Thunder"?

Bradbury initially creates suspense when one of the Time Safari officials tells Eckels that their company does not guarantee his safety during the expedition. Eckels's expedition to the distant past, where he will hunt a prehistoric Tyrannosaurus Rex, is suspenseful and provides an element of danger to the plot. Travis, the Safari Leader, adds to the suspense by informing Eckels that six Safari Leaders and a dozen hunters were killed last year. The reader senses that Eckels is risking his life for the chance to hunt a deadly Tyrannosaurus Rex.

Once the hunters and Safari guides travel back in time, Travis proceeds to lecture everyone on the importance of staying on the path and obeying every directive. His emphasis on obeying the rules and not altering the environment increases the suspense and foreshadows Eckels's tragic mistake. The tension continues to rise as Travis elaborates on how one mistake can dramatically alter human history, which makes Eckels's expedition even more daring and uncertain. Bradbury continues to increase the suspense when the hunters hear the sound of thunder, which is actually the sound of the Tyrannosaurus Rex stomping through the forest. Bradbury's description of the horrific, massive dinosaur and Eckels's impulsive reaction intensifies the mood of the story as the reader wonders if Eckels ran off the path.

The suspense continues to rise once Travis discovers that Eckels did not remain on the path and could have possibly altered the future indefinitely. Knowing that Eckels left the path, the reader is excited and nervous to discover if Eckel's accidentally altered human history when they travel back to the present in the Time Machine.

How does "A Sound of Thunder" create an element of surprise?

The short story "A Sound of Thunder" has a huge element of surprise  at the end of the story.

In the beginning of the piece, the reader learns of a guided safari company that allows people to travel back in time to hunt dinosaurs and other animals that are currently extinct; however, this is not without planning.  Travis, the guide, states that they only choose animals that are about to die because

"we don't want to change the Future. We don't belong here in the Past. The government doesn't like us here. We have to pay big graft to keep our franchise. A Time Machine is finicky business. Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species."

He continues by explaining that even killing a small rodent could inadvertently change history, and therefore, the future. 

The reader also learns, almost in passing, that a major election has just taken place, and, luckily, a man named Keith won: "We're lucky. If Deutscher had gotten in, we'd have the worst kind of dictatorship. There's an anti everything man for you, a militarist, anti-Christ, anti-human, anti-intellectual. People called us up, you know, joking but not joking. Said if Deutscher became President they wanted to go live in 1492."

Once the safari takes place, Eckels succumbs to his fear and runs off the path provided by the company that protects the integrity of the environment of the past. Travis, one of the safari leaders, is livid and even threatens to leave him in the past. Travis knows that it's possible history has changed.  Lesperance, the other safari leader (whose name means "hope"), is hopeful that nothing has changed.

Once they return back to the present, it's immediately clear that something has changed, and they quickly learn that Deutscher has now won the election. The actual surprise is then revealed by saving an important detail until the very end, until after the characters and the readers see that a change has definitely occurred:  

Eckels felt himself fall into a chair. He fumbled crazily at the thick slime on his boots. He held up a clod of dirt, trembling, "No, it can't be. Not a little thing like that. No!"

Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and very dead.

"Not a little thing like that! Not a butterfly!" cried Eckels.

It fell to the floor, an exquisite thing, a small thing that could upset balances and knock down a line of small dominoes and then big dominoes and then gigantic dominoes, all down the years across Time. Eckels' mind whirled. It couldn't change things. Killing one butterfly couldn't be that important! Could it?

By saving this detail until the end and keeping it from the readers and the characters, the surprise is created.