The main characters of Juno and Catcher in the Rye , Juno and Holden respectively have one thing in common: They cannot find their roles within society, among their peers, or within their families. Circumstances have made them see things from a very diverse perspective than that of children of their same age. Juno's circumstance is her teenage pregnancy: A life changing event by all means. Holden's issue comes as a result of the death of his little brother to Leukemia: Also another life changing event.
Clearly, both characters are struggling to find a way to surpass a rough spot in their lives. Additionally, their personalities are different in that they are socially isolated, a bit inept, and what could be categorized as awkward. They are, in essence, two lonely kids who are analyzing their roles. That is why they are similar.
I agree with some of the comments made above. Both protagonists in these texts seek authentic relationships, and express their frustration with the inauthentic relationships that they have and form. However, I would argue that whilst Holden does seem to have reached a turning point in his life through his relationship with Phoebe at the end of the novel, the turning point is much more clearly defined in Juno, as she seems to come to terms with the "fakeness" of people and processes it, still choosing to live her life her way in spite of the various pressures that are placed on her.
It is an interesting comparison. I think that one powerful connection between both the film and the book is that they both feature protagonists who are not "phonies." Holden strives to be quite authentic, rejecting the hypocrisy that he sees in society. Holden desires to be a true individual, distinctive from his world. This makes him unique and different from others. Juno is much the same. She is as intense on being an individual and refuses to acquiesce to what society tells her. Juno seeks for authenticity in others, as she demonstrates it herself. Both characters criticize their social orders and the people in it for inauthenticity. I do think that a major point of differentiation would be that Holden does not face the challenges that Juno does and that she possesses much more of a clear emotional compass whereas Holden's is more obscure.