Perhaps the most important thing George tells Slim about the Weed incident which had not been revealed before is this:
"I was jus' a little bit off, and I heard all the yellin', so I comes running ..."
This means that George didn't really see what Lennie had started to do to the girl and that he couldn't have gotten any information at all from the girl, from the law enforcement officers, from any spectators, or from the men who were chasing them. George and Lennie had to start running immediately and then hide in an irrigation ditch until it was safe to leave town. As far as what motivated Lennie to approach the girl, George had to get that story entirely from Lennie. And it has been shown that Lennie lies to George all the time. It seems likely that Lennie was sexually attracted to the girl--possibly without even realizing it--and that he was trying to tear her dress off. When the girl reported to the local sheriff that Lennie had tried to rape her, she may have been telling the literal truth.
Heretofore, Lennie has been interested in soft little animals--mice, rabbits, and puppies. But it seems apparent that he is developing an interest in human females. When Curley's wife first appears at the bunkhouse door:
Lennie's eyes moved down over her body, and though she did not seem to be looking at Lennie she bridled a little.
After she leaves, Lennie says:
"Gosh, she was purty."
George seems to understand what happened in Weed after Lennie kills Curley's wife in the barn.
"I should have knew," George said hopelessly. "I guess maybe way back in my head I did."
What George should have known was that Lennie is becoming a menace to women. He should have known that Lennie would have raped that girl in Weed if she hadn't screamed for help, and that he probably would have raped Curley's wife if he hadn't killed her in the attempt. George realizes that he can no longer protect Lennie because he can't be watching him every minute of the day and night. The incident in Weed gets started when George is away from Lennie, and the incident in the barn occurs when Lennie is alone with his puppy. George can't continue to control Lennie because he can't be watching him all the time and because Lennie is too big for him. He tells Slim that the only way he could get Lennie to release the Weed girl's dress was to use violence:
"I socked him over the head with a fence picket to make him let go."
The time will come when Lennie will no longer obey George. Lennie is changing. He is already showing resistance in the first chapter when he complains about having to give up his mouse and when he threatens to run away and live in a cave by himself. And if George tries to use violence, Lennie might kill him.
The two primary characters in Of Mice and Men by John Steinbeck are George Milton and Lennie Small. George is small and wiry, rather curt in his demeanor. We realize that he has good reason to be rather tight-lipped about his business. Lennie is just the opposite of his traveling companion, and he is anything but small. Lennie is a huge, strong man who clearly has the mind (and therefore the actions and reactions) of a child.
When we first meet them, they are ready to set up camp for the night in preparation to go to a new job tomorrow. Lennie obviously has some childish habits, such as drinking like a dog from the river and wishing he had ketchup for his beans. We soon discover that Lennie also has some more destructive habits, things which have made the men change jobs fairly consistently.
At one point in chapter one of the novella, George rather scolds Lennie for what happened in Weed, the town they just left. Lennie has been hiding (or at least he thinks he has been hiding it) a mouse in his pocket because he likes to pet something soft. Of course George knows what Lennie is doing and tries to get rid of the mouse because Lennie's "petting" has killed the creature.
Lennie is distraught, claiming he just likes to pet soft things, and George explodes, saying:
"Jus' wanted to feel that girl's dress—jus' wanted to pet it like it was a mouse—Well, how the hell did she know you jus' wanted to feel her dress? She jerks back and you hold on like it was a mouse. She yells and we got to hide in an irrigation ditch all day with guys lookin' for us, and we got to sneak out in the dark and get outta the county."
So, we know that in Weed there was a girl who had a soft dress which of course Lennie wanted to pet. Obviously the girl did not know, understand, or appreciate a giant man pawing at her dress, and not surprisingly she raised a ruckus. This hubbub forced George and Lennie to go on the run, escaping before any more trouble could come to Lennie for his misunderstood actions.
In chapter three, the two men have arrived at their next ranch to work, and there is some question about Lennie's abilities to work and communicate. George explains that he is not taking advantage of Lennie and that Lennie is a hard worker, though he is not much of a talker.
Slim, the boss to whom George and Lennie must answer, asks more about Lennie, and George reveals just a little more about what happened to them in Weed. Slim is a reasonable man and seems to have a good understanding and appreciation for Lennie's disabilities after George explains more details.
He tells Slim that Lennie reaches out to pet the girl's red dress and, not surprisingly, the girl "lets out a squawk." This confuses Lennie and he holds onto the dress even harder because he does not know what else to do. The girl continues her "squawking" and Lennie keeps his death grip on her dress. George eventually hears and comes to Lennie's rescue. He says:
I socked him over the head with a fence picket to make him let go. He was so scairt he couldn't let go of that dress. And he's so God damn strong, you know.... Well, that girl rabbits in an' tells the law she been raped.
The men in Weed gather and become a lynch mob, looking for Lennie. To escape these men, George and Lennie take refuge in an irrigation ditch all day before escaping that night.
We learn two new details from this telling of the story: the girl accused Lennie of rape because she was so distraught and Lennie was in danger of being lynched.