Louis Cancelmi (born 9 June 1978, Pittsburgh, Pennsylvania) is an American stage and film actor who studied Theater at Yale College. He is a frequent performer with the Public Theater, appearing at both its Astor Library home and the Delacorte Theater in Central Park (including The Tempest in Shakespeare in the Park, 2015). He is best known for playing Mike D'Angelo in HBO's Boardwalk Empire (2014, 6 episodes), Victor Mateo in Showtime's Billions (2016–2023, 45 episodes), Sally Bugs in Martin Scorsese's The Irishman (2019), and Kelsie Morrison in Killers of the Flower Moon (2023). He is married to actress Elisabeth Waterston (m. 2006), whose father is actor Sam Waterston. His filmography includes The Amazing Spider-Man 2 (2014, as the Man in Black Suit), 21 Bridges (2019), The Eyes of Tammy Faye (2021, as Richard Fletcher), Pet Shop Days (2023), Sorry, Baby (2025), In the Hand of Dante (2026, dual roles as Lefty / Guido da Polenta), and The Bride! (2026). On television he has also appeared in Blue Bloods (2015–2016, as Thomas Wilder), The Looming Tower (2018, as Vince Stuart, 10 episodes), A Murder at the End of the World (2023, as Todd Andrews, 7 episodes), The Penguin (2024, as Rex Calabrese), and Government Cheese (2025, as Jean-Guy Prevost, 5 episodes).