I've actually seen this movie before, and read the play. I like it. It's about prejudice and the justice system and it's a very stylized movie. Most of it takes place in one room and in real time, and none of the characters have names. They're just Number 8 and 6 and so on. The 12 angry men are a jury, and they're on an apparently open-and-shut murder case with a death sentence hanging over it. All of the jurors are convinced that the accused is guilty except for one, Juror #8, who thinks that they should at least just talk about it for a while.
The rest of the movie is just people talking to each other, about the case and racism and the concept of reasonable doubt. It's a very, very character driven piece - It must be a great play to be in. The power that this movie has is really incredible when you look at how it's made - It's just twelve guys with no names in a room. It's even more incredible than A Few Good Men (It has more men in its title, for one thing). You can totally tell all of the characters are coming from and what we're supposed to think about them, and all they really talk about is the one case. It's great writing.
And the guy who plays Piglet is in it. As it turns out, he uses his natural voice for that role.