As the film tells us at the start, the Charlestown section of Boston is the bank robbery capital of the U.S. It also gives a quote from a Charlestown resident stating that the town was a great place to grow up, but it ruined his life. The characters are products of this environment. It defines them whether they like it or not.
Doug MacRay (Ben Affleck, GOOD WILL HUNTING) is the brains of a bank robbery crew. It's sort of the family business; his father Stephen (Chris Cooper, AMERICAN BEAUTY) is serving time for robbery. During the latest heist, his best friend James Coughlin (Jeremy Renner, THE HURT LOCKER) takes an unnecessary hostage in petrified assistant bank manager Claire Keesey (Rebecca Hall, VICKY CRISTINA BARCELONA). Doug feels bad about the whole thing, so he follows Claire to make sure that she is okay. It also doesn't hurt to know what she knows.