An example of the atmosphere of fear and suspicion is George’s fear of what Lennie is going to do, and what kind of trouble he will get into.
George and Lennie live in constant fear, because George never knows what Lennie is going to do and Lennie is always getting into trouble.
"Run us out, hell," said George disgustedly. "We run. They was lookin' for us, but they didn't catch us." Lennie giggled happily. "I didn't forget that, you bet." (ch 1)
They were run out of Weed because Lennie touched a girl’s dress. Lennie does not know what he is doing is wrong, and often does harm when he does not intend to. George worries about Lennie because he can't predict what he will do.
In addition to the nice example provided by litteacher8, Of Mice and Men is rife with instances of characters expressing suspicion and creating (and/or discussing) situations of intimidation and fear.
When George and Lennie are assigned bunks in the bunk house, George wonders aloud about why the previous man had left and whether or not his bed is infested with bugs. The bug poison he finds leads him to suspect that things are not right with the situation in the bunk house.
He interrogates Candy and checks the mattress before he finally "seemed satisfied" that the situation was legitimate or acceptable.
Following this moment, the boss suspects that George and Lennie are cheating him and lying to him. So he interrogates George.
"What you trying to put over?"
The boss is suspicious of the men's friendship and thinks that George is running some kind of con on Lennie (or running a con on him (the boss)).
"Well, I never seen one guy take so much trouble for another guy. I just like to know what your interest is."
George lies and assuages the boss's worry, but this scene too contributes to the atmosphere of distrust and helps to further establish the idea that the economy of the ranch is one of exploitation with both sides attempting to gain the upper hand wherever possible.
Curley's character is largely defined by his repeated attempts at intimidation. He threatens George and Lennie when he first meets them and continues to act aggressively toward Lennie.
Additionally, the scene in the stables where Lennie, Candy and Crooks are accosted by Curley's wife helps to further the atmosphere of mutual distrust and mutual fear. Curley's wife threatens Crooks, insinuating that with an easy lie she can have Crooks killed, and Candy threatens Curley's wife. These threats come after Crooks has already baited Lennie by suggesting that he should be afraid that George has left him on his own.
From these examples, we can see that fear, intimidation and suspicion are woven into Of Mice and Men in such a way as to become a dominant element of the work.