When you've got a good job and a world-famous girlfriend, you've got it all, right?!? Forgetting Sarah Marshall is a Coming of Age story told well and acted better. Peter Bretter (played by Jason Segal and also the movies writer) is a man too comfortable, sitting on his couch-world, to grasp that everyone must grow up and in the process, change or people will leave him behind.
Peter writes/performs the musical score to a successful TV drama in which his girlfriend, Sarah Marshall, is the lead actress. Sarah leaves Peter, and thus starts Peters journey with the help of friends, to realize that he didnt really like the life he was living. They help him forget Sarah Marshall to grow and change his life for the better.
Peter's half-brother (Bill Hader as Brian Bretter) is the reflection Peter sees in the mirror and is reluctant to embrace --in a relationship with responsibilities to a partner that goes beyond physical presence. [Interesting to note the evolution of Mr. Segal from his supporting role as Jason in 2007s Knocked Up another coming of age movie for 20-somethings...the movie is filled with internal compare/contrasts to itself and the Actor's other works and real-life asperations. How much is intentional and how much is coincident is left to the viewer, but this is a smartly written adult comedy.]
Kristen Bell as the title's "Sarah Marshall" does well with the material -- her character is mult-dimensional as she is both the villain and the victim. While her character emotionally hurts Peter, she has cause as revealed in a fairly depicted series of flashbacks. [It is interesting to note, Sarah's desire to break out from being a successful TV actress to movies is an interesting simile to Peter's desire to break out from being a successful TV mood musician to aspiration to become a Rock Opera composer and Ms. Bell plays it well from the heart as if she also longs break out of the Veronica Mars role to her first leading actress role.]
But it is not just Peter that Sarah is running from; she has run into the arms of Aldous Snow (Russell Brand), a famous lead singing rock star. Snow almost steals the movie, with his philosophical musings and ability to play the guy you want to hate but wish was my best friend despite stealing my girlfriend role perfectly. In some ways Peter sees Snow as the worldly guy he wishes he was, but Peter realizes that he cannot be like Snow and force the world to take him as he is and be successful, but rather adapt to the world in his own terms to succeed. [Of course there are parallels when Snow plays to the crowd and to his girl and when Peter plays to another crowd and his girl...the men are more similar on stage than they realize, which may be why Peter likes Snow.] At the critical point, it is Snow's unwillingness to grow that has him to move on and gives the opportunity he thought he sought -- the chance to reunite with Sarah.
However, prior to getting to that point in the story, Peter gets off his real and proverbial couch to meet a series of people whom help him "forget" Sarah. Pride, self-worth and compromise are explored well without bogging down nor being too serious. Central to Peter's growth are Chuck (Paul Rudd) and Rachel Jansen (Mila Kunis). Chuck is the surfing instructor who sees the hurt behind Peters eyes and teaches him to stand on a surf board (another analogy for standing up for himself/getting back up when one is knocked down). While played at times for laughs, there are real life-lessons in what Chuck, Rachel and the other characters offer Peter. Ultimately, Peter stands up and realizes that his affection for Rachel is real and worth the risks to help out a friend. Through them, Peter breaks out and becomes the man he learns he wants to be and achieves the personal satifaction and professional success he seeks.