Brian De Palma is known as a director who likes to let the blood flow, but he does more than that in this film. This film outdoes his earlier Scarface in nearly every way - namely, the actors have more of an opportunity to act, the score is better, the action is less drawn-out, etc. One notable difference in his style is his reliance on the perspective of the Untouchables, and not of Capone's men. In giving more screen time to the good guys, he allows some beautiful shots to enter the foray; but don't think that he has completely abandoned violence. It is here as well, and plays a vital role in the film, but is pushed to the background most of the time.
Kevin Costner (in one of his early roles) stars as Eliot Ness, the leader of the so-called Untouchables, a branch of the government responsible for nabbing the bootleggers in 1930s Chicago (and elsewhere, though the Windy City is the sole focus here.) He is very convincing as Ness, and De Palma does a great job at showing his early rejection, his first triumphs, and his ultimate success. De Palma also makes Ness' work seem more personal, because he has a wife and children for whom to care.
Sir Sean Connery gives one of his best performances in recent times as Jimmy Malone, who somewhat reluctantly joins Ness in his crusade against the bootleggers. As a hard-nosed cop with decades of experience, he is essential to the Untouchables and their mission. His performance understandably earned him an Oscar.
The other actors are also well-cast, especially Robert De Niro, who is famous for his mobster roles. His part as Al Capone does not get much screen time, but the time that he is in the picture is perfectly captured.
Special mention must be made of the score by Ennio Morricone, surely the greatest and most prolific composer of film music. As in most of the films that he scores, the music becomes an essential part of the work; not only do certain scenes become firmly implanted in your mind, but also his music for each scene. He received an Oscar nomination for this film, but did not win (he has never won, despite six nominations.)
I cannot recommend this film highly enough. It is unique in its perspective, the images are not excessively violent, and the score is outstanding. Consider this De Palma's good-guy gangster film, and a must-see.