I need a thesis sentence on Macbeth and The Catcher in the Rye to show how a minor character from each text helps to develop the main character.
There are certainly plenty of examples in these two texts as to how minor characters are used to develop major characters. Consider for example how the prophecy of the witches has such a massive impact on Macbeth. Note how Macbeth greets the news that he has been bestowed with the title of the Thane of Cawdor:
Two truths are told
As happy prologues to the swelling act
Of the imperial theme.
Macbeth clearly feels that the completion of the first prophecy indicates that all of what the witches tell him will be true, including the "imperial theme" of his own rise to the position of king. This, arguably, sets him on his path to regicide and self-destruction as he kills Duncan and tries to establish his position.
As for Holden, Phoebe would be a great character to examine. It is Phoebe after all that seems to break Holden out of his cycle of wandering aimlessly around and helps him to look for some kind of medical support. Note how the sight of her at the end of Chapter 25 has such a profound impact on Holden:
I felt so damn happy all of a sudden, the way old Phoebe kept goin around and around. I was damn near bawling. I felt so damn happy, if you want to know the truth. I don't know why.
It is the picture of Phoebe going around the carousel that seems to help Holden accept help and to move on in his life. This simple picture of pure happiness and pleasure seems to be the antidote to all the "phoniness" that Holden sees in adult life, and clearly moves him.
A great thesis statement that can be used to answer this question would therefore be something along the lines of:
In Macbeth and The Catcher in the Rye, both Macbeth and Holden are developed as characters through their interactions with minor characters.
Such a thesis statement gives the opportunity to look at a range of minor characters and how they help develop the central protagonist.