Paul Giamatti did not receive an Oscar nomination, despite being nominated for a Golden Globe, Sag, while also receiving many critical citations and winning The Independent Spirit Award and NYFCC, for portraying Miles in Sideways.
Giamatti certainly seems well cast in the part, and delivers in quickly establishing Miles as a very certain type of man. Giamatti rightly portrays Miles as basically always at least semi-depressed in most every scene of the film. Giamatti does not show the depression to be something overwhelming that would necessarily even raise any concerns though. Giamatti is very effective in portraying it as a very much lived in depression that resides in Miles. He always seems at least somewhat down in his whole manner, and his semi down beat manner speaking. Giamatti hits just the right note with it because it properly denotes the recent history of Miles's life, but he handles it in a way that although noticeable it would never really ever cause worry. Giamatti makes seem as though a character trait simply due to the time in which Miles has had it. He also does quite well though to make it truly something dynamic in Miles in that the intensity of Miles's general malaise properly changes depending on the situation.
Normally Giamatti shows Miles's distress being there but quiet, some circumstances make it almost disappear, and of course being reminded of his wife Giamatti shows it as something quite explosive. The only times where it really seems to almost go away are whenever Miles talks about his love and knowledge of various wines. Giamatti handles these moments very well by showing Miles when in his element and in his passion that he almost forgets his personal plight for a moment. Giamatti actually technically treads the line of being unlikable here as he definitely brings a certain smugness to Miles when he is offering his sage advice on proper wine making and drinking. Giamatti avoids being unlikable though because he is able to gain sympathy through every situation by making Miles genuine as the type of guy he is. Miles is far from perfect, and he probably is the cause of much of his suffering, but Giamatti only ever shows this as the type of flaws someone simply has.
One of the highlights of the film is the unique chemistry between Paul Giamatti and Thomas Haden Church as Miles's friend Jack. Although they are friends the two do not create their relationship as some truly great friendship between the two men. It's a friendship but a very particular and realistic one between the two. They are not alike in many ways with the often upbeat Jack contrasting a great deal from the very much downbeat Miles. It is not the similarities that bring them together but rather the one sorta allows the other one to be. On the one hand Jack is a philanderer and Miles acts as almost a non-moral compass as Giamatti plays these scenes with only ever suggesting that Miles never really stops Jack from being Jack. On the other hand Jack does take Miles's depression in stride while offering constant support to his friend the best he can. It is an unusual friendship to be sure but it is a wholly believable one through the honest chemistry the two share.
Miles personal journey is extreme in moments but rather modest on a whole. As I said before Miles's depression of sorts never leaves him wholly or at least never permanently, not even in the final moments when perhaps there is hope for him. Giamatti though weaves kinda a tapestry of the highs and lows of his personal journey. Giamatti is exceptionally natural about as he allows there to be a flow and not be a flow at the same time. There are moments of happiness inter spliced with sadness that seems always honest for his character. For example when he finds a potential love in a woman Maya (Virginia Madsen) there is the sense of things becoming better for Miles yet Giamatti shows that his general sadness in a way drives him to her to begin with. This is a very good performance by Giamatti simply by making Miles seem like just someone you could meet. Even in the more comedic moments Giamatti plays his reactions in a funny yet down to earth fashion. It's a strong leading work that works well in bringing the whole film together.