It's very rare you get a great script just handed to you, or sent to you, by someone you don't know.
I try to research or make up for myself what happened in any character's life. From when he was born until the first page of the script. I fill in the blanks.
When you're under stress as a human being, you behave oddly and your relationships with people become strained.
I know people who prepare their roles in such a way that they technically look ahead and memorize their gestures, and then they stick to it. Those that are technically proficient enough can make it seem natural, but they do that and don't really take in what other people are doing.