In chapter 4, Lennie walks into Crooks's room uninvited and Crooks sees an opportunity to make Lennie feel bad about his situation. Crooks begins to upset Lennie by hypothetically asking what he would do if George did not return to the ranch and left without saying goodbye. Crooks continues to question Lennie about the possibility of George not returning and leaving him alone to fend for himself, which upsets Lennie to the point that he becomes aggressive and threatening towards Crooks. Crooks is aware that George leaving is Lennie's worst fear, which is why he brings it up.
After Lennie accidentally kills Curley's wife, he flees to the secret location on the banks of the Salinas River and begins to hallucinate. During Lennie's second hallucination, he speaks to a gigantic rabbit, who also tells him that George will leave him because of the terrible mistake he made. Both Crooks and the gigantic rabbit threaten Lennie with his worst fear, which is George leaving him alone to fend for himself.