... (which, if you've read the other reviews here, you know is a massive understatement).
Let's start with the two obvious stumbling blocks to loving this rather astonishingly good and (for some) altogether life-affirming and lovable movie.
-- The main character is pretty much a jerk.
-- There's no plot.
Now, realize: the main character is *supposed* to be unlikeable. And the movie does not try to have a plot. And you might wonder, how can a movie that starts with these two conditions possibly be any good, let alone some be some kind of masterpiece? So let me explain.
Roger Greenberg (Ben Stiller in the performance of his life) is by no means completely unlikeable. He's quite funny, an adept verbal wit (there is no situational comedy in this movie at all; it's funny in places because Greenberg is often funny). He has very firm and uncompromising moral ideals. He cares deeply and responsibly for animals. What makes him unlikeable is that he sometimes, unexpectedly, treats people, especially people he cares for, like crap -- he uses that verbal wit like a blunt knife and goes on unwarranted tirades. And he has almost no insight into himself (occasionally the movie is very funny at his expense.) You know that guy in your family who is fundamentally, deep down, a very good person, but is all too often an incredible pain and aggravation to be with? That's Greenberg. He is, in short, the sort of person who is much easier to love than to like.
We are told from the outset that he has, in fact, just been released from a mental institution, and this is one of the least clichéd and most fully-rounded portraits of a seriously mentally ill person in recent cinema. We watch as he works really, really hard to sabotage any hope he has for a happy life.
He meets and is immediately smitten with a woman (played terrifically by Greta Gerwig; it's nearly as much her story as his) who has her own set of neuroses, but ones that are much more ordinary than his. We watch their relationship play out over a few months exactly as it might in real life, without any set of causally related events to provide an engine of plot.
So what narrative tension is there? Why keep watching? Very simply, if you like Roger Greenberg enough to have empathy and sympathy for his plight, if you can see past the sarcastic surface to the deep pain underneath, if you can recognize a little bit of yourself in him, then you are going to root for him to get his act together, to cut all that nonsense out, to start to understand himself --in short, to start to recover some sanity, to find the first cobblestones of the path to some kind of happiness.
Well, that's what happens, and it's wonderful to watch. (That it's possible to watch the movie and not notice it happening is obvious from some of the reviews here, including one that criticized the ending as incomplete; in fact, this may have the best and most perfect last line of any movie I can think of.)
If you didn't much like _Sideways_ or _Up in the Air_, I can guarantee that you will hate _Greenberg_ (I mean, imagine those movies, but with no plot, too!). If you loved those movies -- if you have no problem watching emotionally stunted people slowly grow up -- then you'll probably love this, too, and you may even love the way that the lack of conventional plot makes the movie completely realistic and hence especially powerful emotionally. When I first wrote this review, I ended by saying "I can't wait to see it again and I suspect I'll watch it many times." I've now seen it twice more and, indeed, it only gets better.
