Mystic River is a decent enough film and is strong in many aspects but it has some problems involving a few scenes being too over the top and the plot being a little to convenient.
Sean Penn here plays Jimmy Markum a street smart Bostonian who loses his daughter to a mysterious killer, and then goes to extreme measures to get to revenge for his daughter. Penn performance is not all bad as Markum so I will begin with the problems with the performance and though go to some of the strong moments later.
The major problem with his performance is any scene which he is required to show grief over his Daughter in the film. In every single scene involving this he goes over the top and unfortunately hams it up. He never sounds believable doing this to me instead it looks like look at me "I'm Acting". He never had the right style in theses scenes he always seems over the top no matter what. Every time he says "My Daughter" it just rings false to me. Even later when he does these little facial gestures with his mouth he tries to be subtle in showing his grief but again he comes off as far too forced and actory. He basically is too calculated in these scenes and never seems to off the real raw emotions which he needs.