What are some passages in Harper Lee's To Kill a Mockingbird that signal Jem as being either a leader or a follower?
Some examples of Jem acting as a leader in Harper Lee's To Kill a Mockingbird can be seen in the various games he invents for himself, Scout, and Dill to play together. One particularly good example is seen on the day he announces the new game they will play:
I know what we are going to play. ... Something new, something different. ... Boo Radley. (Ch. 4)
Jem further explains they will each play a different role in the Radley household and act out the rumors and myths they know concerning Arthur Radley's life, whom the children call Boo Radley. Scout is assigned the role of Mrs. Radley; Dill is to play "old Mr. Radley"; and Jem plays Boo. Jem continues to act as a leader when he finds a way to rebel against his father. As soon as Atticus realizes what the children are playing and expresses hope that they aren't playing a game, Jem asserts that Atticus didn't actually tell them to stop playing; therefore, they can still play if they "simply change the names of the characters" (Ch. 5).
In contrast, Jem acts as a follower when under the influence of Dill. Jem's decision to play the Boo Radley game was the result of Dill's curiosity about what Boo looks like. The plan was to provoke Boo to come out of his house so Dill could finally see him. When the game and their other attempts fail, Dill thinks of one more approach that Jem willingly agrees to--trespassing on the Radleys' property at night to try and get a look at Boo through a window. We can tell trespassing is Dill's plan, not Jem's, because it's Dill who gives the signal to Jem to start their adventure, stretching, yawning, and saying, as Scout describes, "altogether too casually, 'I know what, let's go for a walk'" (Ch. 6). At Dill's cue, Jem gladly follows along and tries to persuade Scout to go home; when she refuses, Scout feels obligated to follow along as well.