The night before they arrive at the ranch, George warns Lennie to be quiet when they meet their new boss because
If he finds out what a crazy bastard you are, we won’t get no job, but if he sees ya work before he hears ya talk, we’re set. (chapter 1)
This quote highlights Lennie’s two most salient, and opposing, traits: his mental backwardness and his huge physical strength. Throughout the book we see how he talks like a child, repeating words, asking George endless questions, excitedly looking forward to the time that they'll have their own place and he will look after the rabbits. George therefore is worried that Lennie’s lack of intelligence will be revealed in his speech and will jeopardise their chances of being hired.
However, Lennie's physical capabilities are just right for the kind of work that is available at the ranch. Slim, after seeing him in action, testifies to this:
Maybe he ain’t bright, but I never saw seen such a worker. He damn near killed his partner bucking barley. There ain’t nobody can’t keep up with him. (chapter 3)
Lennie’s fearsome physical strength, as well as being an asset, can sadly also turn into a liability. We learn that he and George were forced to flee from their last job after a girl accused Lennie of rape when all he wanted was to feel her pretty red dress. This foreshadows what will happen at the end of the novel, when Lennie accidentally kills Curley’s wife. His sheer strength, combined with his simple-mindedness, ultimately has tragic consequences.
Why did Lennie's mice always die?