When Eckels confronts the dinosaur, his initial reaction is one of disbelief. He says that it is "impossible" to kill this dinosaur and that they were "fools" to come. In other words, Eckels has seen the size and scale of the dinosaur and has lost his nerve.

When the dinosaur raises itself up, his reaction becomes very fearful. He wants to return home immediately and admits that coming on this trip was a mistake:

"I’ve met my match and admit it. This is too much for me to get hold of."

Travis agrees to let Eckels go home and instructs him to turn around and go back to the time machine. Unfortunately, however, Eckels is paralyzed with fear, and when he finally gets his legs to move, he stumbles off the path and into the jungle.

It is only later that Eckels realizes the error of this action when he learns that he has killed a butterfly and radically altered the future.

In Ray Bradbury's story "A Sound of Thunder," when Eckels encounters the dinosaur, the normally garrulous man reacts with stunned surprise and fear.

He had been excited to hunt the great beast, having taken down large animals before as an excellent huntsman, but the sheer size of the Tyrannosaurus Rex makes Eckels fairly quake with fear. Even though he'd paid a steep price for the mission to kill this dinosaur--$10,000--upon actually seeing it, Eckels quickly decides to give up and run away:

"Why, why," Eckels twitched his mouth. "It could reach up and grab the moon." ... 

"It can't be killed," Eckels pronounced this verdict quietly, as if there could be no argument. He had weighed the evidence and this was his considered opinion. The rifle in his hands seemed a cap gun. "We were fools to come. This is impossible."

Eckels's reaction adds to the tension of the story, illustrating the incredible power and terror of the dinosaur. This is a man who has hunted and killed wild boar, elephants, buffalo, and even tigers. But the T. Rex is too much. It scares Eckels. He goes back into the time machine and just lies there, shivering, while the other men take down the T. Rex.

His foolish reaction also foreshadows the foolish error he's about to make. In his haste to hide himself in the time machine, Eckels steps off the path, changing history forever.