In the first chapter Steinback characterised George as, small and quick, dark in face with restless eyes and strong, sharp features. I have investigated this description in to detail and I believe that the description of him also can show his strength and weaknesses.
In chapter one when they come up by the river Lennie sprints to the water to drink, George warns him that he should never drink water that isn't running then George leaned over the river and tasted to water saying, "I ain't sure it's good water". Being almost satisfied. George seems to be keeping an eye out for everything around Lennie and himself; he is very protective and he is an outlook for trouble.
Shifting further into the chapter George then notices that Lennie had his hands in his pockets, and asks what he has. It is a dead mouse that Lennie had kept to pet with his thumb as they walked. George than takes the mouse away from him, and explains that they are going to work on a ranch like the one in weed from where they came and tells Lennie not to speak when they get to the ranch for their boss will think Lennie was insane. And to make sure Lennie tries his best of keeping the promise of not speaking George assured Lennie could have another mouse if he takes care of it.
When they then meet the boss George lies to the boss by telling him that he is Lennie's cousin, he reinforces the suspicion that there is something suspect about their friendship. The boss cannot understand that Lennie and George would have any concern for each other unless if they were bound by family connections. We know that George is a childhood friend to Lennie and takes care of his mentally disabled friend. That tells us that Lennie is like a brother to George.
Before meeting the boss, during and after meeting the boss George shows to be careful and controlled; formulates a plan for every situation and acts to prevent any dire occurrence. This must mean there had to be a past experience and he must have been so ashamed or disappointed of what Lennie had done.