The characters of Ali, Gordon, Pam, and Sue in the Life Support meeting are named after HIV-positive friends of writer Jonathan Larson. The real "Ali" was Alison Gertz, a young woman who contracted HIV in the early 1980s and became well-known later in the decade as an AIDS activist. A TV movie about her life, Something to Live for: The Alison Gertz Story (1992), starred Molly Ringwald as Ali.
Edin Espinosa, who plays Maureen, famously played Elphaba on broadway, following after Idina Menzel, who originated the role of Maureen on broadway.
Tracie Thoms, who appeared as Joanne in the final months of Rent, also appeared in the film version as the same character.
Rodney Hicks (Benjamin Coffin III) and Gwen Stewart (Mrs. Jefferson/others) are the only members from the original Broadway cast to be part of the closing cast for the show and film. Stewart returned to the cast in the same role she played in the original cast ("Mrs. Jefferson, woman with bags, and others," including the solo in "Seasons of Love"). Hicks also returned to his original cast role, in a way--when the show first opened, he had played "Paul, a cop, and others," and had also been the understudy for the role of Benjamin Coffin III, which was then usually played by Taye Diggs.