Alan Arkin portrays the crusty old grandfather, who since it is an independent film of this sort is also a Heroine addict. The fact that he is a heroine addict really does not play into his performance, it plays into his character's fate, but Arkin really never portrays this as really all that important of a facet of his character. After all it never even shows the Grandfather after he has used the heroine, it only in fact shows him use it once, and we do not see any after effects. The point of his character really just comes in with him being a crusty old man apt to say one liners, and go off on rather salty tirades when he chooses to.
I must say Alan Arkin is quite good at the role set for him. He appropriately crusty, and actually quite amusing in the way he unabashedly portrays Edwin's various opinions as loudly as possible at times. Arkin is always effective in the role, and does perfectly realize this sort of character quite well. He never apologizes for the character, not once, which makes his performance far more effective than if he tried more visibly to make his character more likable in these moments. Arkin though actually does make Edwin actually fairly endearing, since he always makes it clear that Edwin merely is telling it the way he sees it.
Arkin makes all of Edwin's tirades quite enjoyable actually, as are his quick well timed little reactions or insertions into other conversations. It is an expertly done performance by Arkin actually because he really does play it straight for the most part. Yes he is meant to be humorous, and Arkin brings humor out the performance, but he always presents it through a realistic portrayal of a old man like Edwin. A part like this very easily can seem overwritten if the actor had overplayed the role, but Arkin never overplays Edwin finding just the right tone for him through his whole performance.
Arkin despite giving a largely comedic performance also does find a great deal of heart in his performance actually particularly in regards to Edwin's relationship to his granddaughter Olive. The scenes of Breslin and Arkin together I am sure is what helped both secure their nominations, as their is a surprising degree of warmth in their moments together. Arkin always shows that Edwin genuinely loves his granddaughter and always wants what is the very best for her. Their final scene together as well as when he convinces her to eat the ice cream despite the rather cruel objections of her father (Greg Kinnear), Arkin realizes a honest sweetness that actually is quite moving.
It is actually quite a challenge for Arkin to connect both his scenes of humorous crustiness, and his scenes of Edwin trying to be a good Grandfather and even father. His scene where he tries to encourage his son, is all terrifically handled by Arkin showing that even after all that flak he gives his son he still does love him. Arkin never fails to combine both sides of his character into one single characterization of this man. It would have been very easy to overplay the comedy, and completely forget the heart of his character, but Arkin succeeds in every respect. My only complaint was I wish he was in the entire film, but alas this is still entertaining work from Alan Arkin.