Kevin Bacon did not receive an Oscar nomination for portraying Detective Sean Devine in Mystic River.
Bacon takes an understated method through his entire performance. Even his Boston accent, which is present, is very much downplayed, but frankly is far more natural than it is rest of the actors. Bacon due to the fact he really does not get that many personal scenes, has to create his character as Sean and his partner (Laurence Fishburne) investigate the murder of Jimmy Markum's (Penn) daughter. Bacon has a few moments to himself mostly being when Sean is called by his wife over the phone, a subplot that frankly could have been dropped as the payoff does not work all that well due to the actress playing his wife, but for the most part Bacon is forced to find his character while moving along with the plot at a rapid pace.
Bacon is quite effective in portraying Sean in this way actually as he tries to find the killer while trying to keep his personal feelings behind him despite being childhood friends with both Jimmy, and Dave (Robbins) who slowly becomes a suspect. Bacon underplays the role and fittingly for the character. He shows that Sean very much has a job to do, and is quite intent and doing it. Nevertheless Bacon conveys well the personal connections Sean does feel due to his attachment to the case. He is always very collected, but through subtle reactions he portrays the way the events due effect Sean, Bacon's very small response to seeing the murder victim is Jimmy's Daughter is particularly well handled.
Throughout the film Bacon is steadfast in his portrayal that Sean is a good a detective. He believably portrays Sean as a confidant officer with an underlying passion to solve the case, and even with his personal connections he will still do his very best to find the killer. The doubt and hesitations though are equally well placed by Bacon as Sean does struggle to wave his personal thoughts with the case when some evidence points that Dave may be the killer. Bacon again keeps his performance very much contained in terms of the emotions shown, but he still powerfully brings the struggle in Sean to life. It is not something that overwhelms, but Bacon more realistically shows it to be something that presses on in the back of his mind less at some points, but heavier when he is faced more directly with his past.
This is strong work from Bacon as he still does show that even Sean has
never forgotten watching Dave being taken away by pedophiles when they
were kids. He is not at all overt in his depiction of this very much
again establishing it as something that very much sits within Sean, it
is not always on his mind, but Bacon shows that he can't ever forget it
either. Bacon is great in this role because he stays so calm in the
part, and stays as someone you can more easily follow along than Penn
with his screaming. Bacon is the one who keeps the film grounded with
his far more down to earth portrayal of Sean. Bacon despite being very much put behind the other actors in terms of the types of scenes he is given, he actually gives my favorite performance in the film by taking this fair less flamboyant approach to a story that never calls for it.