Variety described Holkenborg's score as a departure from his previous work, "In a complete about-face from his adrenaline-pumping Mad Max: Fury Road soundtrack, Dutch composer/producer Tom Holkenborg (aka Junkie XL) supplies an elegiac orchestral score that perfectly complements the film's desperate, wintry mood." After first seeing the film, Holkenborg was feeling anxious; his hands were sweating and his legs were shaking because he was so impacted by the unsettling emotions of the film. He collaborated closely with director Scott Cooper, aiming to enhance the director's vision by musically exploring the humanity and depths of the reprehensible characters at the center of the story. "In a lot of gangster pictures, you don't feel deep wells of emotion that I like the audiences to feel in my work. Tom understood that I was making a film about humans who happen to be criminals as opposed to criminals who just happen to be humans. In doing that, he understood the sound that would take the film to its highest level and be one that would unsettle the audience, unnerve them, but also move them and stir them," explains Cooper.
Tapping into the heart of the film with an orchestral score featuring piano, strings and woodwinds, Holkenborg created a dark theme for Bulger punctuated by a reoccurring low frequency pattern played by an ensemble of cellos. Mirroring Depp's character in the movie, the composer transitions the vibe of Bulger's theme by shifting the instrumentation and changing the key of the harmony for the scenes with Bulger and his wife. Holkenborg also incorporated sound design from a piano by beating the piano strings with a stick to create a deep, ominous sound for one of the most evil, notorious gangsters in American history. "Jimmy (Depp) is such an evil person, and while being so incredibly dark, there is also a lot of nuance to his character. The music needed to not only reflect him at his worst, but also needed to capture the relationship he had with his wife. His relationship with Agent Connelly was also important. I juxtaposed their themes and would use them to play off each other as the characters do in the film," stated Holkenborg.