Paul Giamatti has a rather unique challenge in his portrayal of Pekar since unlike many biography films the real Harvey Pekar appears on screen. This is not just a one scene thing like Tina Turner in What's Love Got to Do With It. Harvey Pekar appears throughout the film even commenting at one point that Giamatti does not really look like him. So Giamatti actually has to stand on instant scrutiny of comparing him to the real man, although he is perhaps helped a little in that he is portraying the man at a different point in his life, but still Giamatti must constantly be standing up right next to the real deal.
Giamatti actually meets the challenge well and there really is not any problems when comparing his performance to Pekar. Although obviously it is not seamless in the sense that he simply becomes Pekar, but he effectively makes it so there is not any extreme disconnect between the real one and the performance. Giamatti is quite good here in bringing into his performance Pekar's mannerisms whether it is his usually furrowed brow, his rather often cross grimace, or his sometimes rather raspy voice. Giamatti handles them all well never being obvious acting, and he utilizes the mannerisms in giving a convincing characterization as Pekar.
The overwhelming aspect of Giamatti portrayal is of course his portrayal of Harvey's seemingly endless contempt and bitterness that also seems to form into a certain depression at times as well. Giamatti portrays this particularly well by bringing across just how intense and prevailing this is in with Harvey at all times. It is not that any of his bitterness ever really goes away from him in any point even when Harvey is relatively happy, Giamatti never loses this part of Harvey for a moment, quite properly showing that this is something that is simply a part of Harvey that he never loses.
Although the bitterness is always a part of Harvey, Giamatti never really makes it downbeat or depressing as it easily could have been. Giamatti effectively finds the humor always within this bitterness and the sardonic slant that is found in Harvey. Giamatti is very good because he is able to bring about the sense of humor in Harvey that really is what created his comic book to begin with. Giamatti is able to be funny, and convey the fact and Harvey himself though he never ever shows that it comes from anything more than his rather extreme form of cynicism.
Giamatti actually makes his cynicism enjoyable in a way, by simply portraying it as the most honest form of Harvey. If Harvey was optimistic, and happy he simply would not be Harvey, and Giamatti expresses that perfectly in his performance. He never portrays there being an actual effort in Harvey when he is talking about something obnoxious in life, it just is the way Harvey deals with things whether it is waiting in line behind an old woman, or his reaction to the Revenge of Nerds, Giamatti always shows that Harvey's says only like how he sees it no matter how others may react.
Although his cynicism never completely wains there are a few warmer moments, although still Giamatti still is fitting in that they are warmer moments for Harvey. Two early on regard his comics when an artist agrees to draw for him, and when his co-workers are amazed by the first issue. In both regards these are quick but incisive moments where we see behind still Harvey's bitter face there is just the slightest happiness and pride in him. Giamatti handles these moment incredibly well because how subtle he is when portrays the softer side of Harvey that actually is made quite sweet.
The most important sides of the softer Harvey are shown with his wife Joyce (Hope Davis). Their relationship certainly is a bit on the abnormal side. Both Giamatti and Davis make it work frankly because they authentically bring out this relationship by being so casual together. Giamatti is very good in these scenes because he so well combines the frustrations of the relationships, but very importantly at the same time does carefully show that Harvey's does very much love his wife. Giamatti finds the highs and the low of their relationship through again his completely honest portrayal of Harvey's complex personality.
This is a very good performance by Paul Giamatti that is an entertaining performance, while being quietly moving at times as well. He never cheats his character though and every where he goes with Harvey is always the same Harvey. Giamatti although portrays a cynical bitter man, he turns Harvey into a very liable cynical bitter man. He even manages to stand up to the scrutiny of being right next to the man he portrays, and does not falter even with this challenge.