In The Perks of Being a Wallflower, we follow Charlie - an introverted boy who is trying to fit in. We follow his life through his journal and are receiving information in first person perspective. We go through his struggles and sympathize with his pain. But what if we were to follow Sam - Charlie's extraverted best friend?
The tone and mood would be entirely different.
Sam's life is much more chaotic and extraverted than Charlie's. By following an extraverted character and seeing their relationship potentially develop with Charlie, we would get an entirely different feeling as compared to following Charlie. Since Sam's life is much more extravertedly-driven, we would experience more characters and more external stimulation, compared to Charlie's calm and internally focused life.
Even more interestingly, we would be able to truly understand how Sam feels about Charlie and not what we are seeing from Charlie's perspective. We would get the absolute truth.
By changing perspective in The Perks of Being a Wallflower from Charlie to Sam, we would experience an entirely different story, filled with much more external stimulation and true feelings.