Why does George want to talk to slim?

After working hours, as the other men play horseshoes outside, Slim and George return to the bunk house. We learn that Slim has allowed Lennie to have one of his puppies. Slim praises Lennie for his incredible work ethic, which leads George to talk about his past with Lennie. The two grew up as neighbors and George took Lennie as a travel and work companion when Lennie's Aunt Clara died. George says that when he first began traveling with Lennie he found it funny to play pranks on him. One day he ordered Lennie to jump in a river even though he couldn't swim and Lennie unthinkingly obeyed. After George fished him out, Lennie was completely grateful, having forgotten that George had ordered him into the river in the first place. After this episode, George decided against having fun at Lennie's expense.

At Slim's insistence, George tells about the episode in Weed that led them to seek work elsewhere. Lennie saw a woman in a red dress and, overcome by an urge to feel the pretty fabric, he stupidly grabbed the woman. The woman fled and told the men of Weed that Lennie had raped her. George and Lennie were forced to hide from a lynch mob and sneak out of Weed under cover of night. Thus, George's conversation with Slim allows him to give the man a heads up and be honest about what brought them there.