Scout's keen sense of observation for one so young is best seen during the trial of Tom Robinson. Although much of the testimony is over her head, and at times she does not understand in what direction her father's questions are headed, she shows that it is not her first time in a courtroom. After Atticus shows that Bob Ewell is left-handed, Jem exclaims that "We've got him."
I didn't think so: Atticus was trying to show, it seemed to me, that Mr. Ewell could have beaten up Mayella. That much I could follow. If her right eye was blacked and she was beaten mostly on the right side of the face, it would tend to show that a left-handed person did it. Sherlock Holmes and Jem Finch would agree. But Tom Robinson could easily be left-handed, too... I looked at him. His back was to us, but I could see his broad shoulders and bull-thick neck. He could easily have done it. I thought Jem was counting his chickens.
Scout was wrong about Tom's left hand, but she was right about Jem counting his chickens too soon. When the jury returned to the courtroom,
I saw something only a lawyer's child could be expected to see, could be expected to watch for...
A jury never looks at a defendant it has convicted, and when this jury came in, not one of them looked at Tom Robinson.
As for her impulsiveness, Scout is quick to use her fists during the early chapters of the book. She tries hard to control her temper as she gets older, but on the night that the lynch mob confronts Atticus, it is not her fists that Scout uses. When one of the men "yanked Jem nearly off his feet,"
... I kicked the man swiftly. Barefooted, I was surprised to see him fall back in real pain. I intended to kick his shin, but aimed too high.
She and Jem refused to leave as Atticus had instructed, trying to guide them away from what he knew could be a violent situation. Instead, when she recognizes Walter Cunningham's father among the group of would-be murderers, she engages him in innocent conversation.
"Hey, Mr. Cunningham. How's your entailment gettin' along?"