There's not another actor alive that could have played Walt Kowalski in "Gran Torino," except for Clint Eastwood. This isn't because the role is deep, though it is, or because there's script convolutions of any kind. The truth is that, had any other actor been filmed growling like a bear in close-up, it would have been so silly that the audience would have been lost beyond the point of repair. Enter Eastwood, the penultimate tough guy. The reason he works in this role, and the reason "Gran Torino" is so good, is because this is how we imagine he functions in his day-to-day life, and have imagined him ever since he played Detective Harry Callahan all those years ago.
The film starts off with the funeral of Walt's wife. He's miserable, because at the funeral his granddaughter shows up in a tube top, his grandsons in football jerseys, and it's evident that his sons, their parents, could care less. His wife's priest, Father Janovich (Christopher Carley) informs Walt that her last wish was for Walt to go to confession, and Walt dismisses him. To Walt, the final straw is when a Hmong family moves in next door. Walt is a Korean War vet, and one of those ornery old men so racist that they can drop a term like "gook" into casual conversation and not even see anything wrong with it.
Walt also lives in the "old neighborhood" of Detroit, which is becoming more of a ghetto with each passing day. One day, Walt sees that a local Hmong gang is harassing the boy next door, Thao (Bee Vang). He doesn't care, until they end up on his lawn. At that point, Walt storms outside, rifle in hand, and demands that they get off his lawn. Thao's hyper-smart sister Sue (Ahney Her) informs him that he's a hero to the neighborhood, which would explain all the meals and flowers being left on Walt's porch.
The film doesn't exactly break any new ground as far as the "old man grows to care for younger kids, learns something about himself along the way" subgenre goes, and so I'll stop synopsis here. What the film lacks in innovation, it more than makes up for in raw emotional impact. Walt realizes over time that the neighborhood isn't the place it used to be, and that it's a lot more dangerous. The key here is that there's no monologue about how he feels old or can't handle the changing times. Walt knows the score, finds it unacceptable and goes about trying to fix it the only way he knows how; he teaches Thao how to be a man, allows Sue to educate him on a foreign culture and violently threatens anybody who he sees as a wrongdoer.
Eastwood doesn't direct himself/portray Walt as anything more than an older version of his past characters. There's no sappiness to be found here; Walt's still a mean old racist at the end of the film, even if he's allowed a few people into his life that he hadn't before, there's no unnecessary character arc to leave the audience feeling good when they leave the theater. He makes a decisive choice at the end of the film that is genuinely heartbreaking, because it seems like it might be the first truly selfless choice he's made in his entire life.