In the biopic, which stars Melissa McCarthy as writer-turned-letter forger Lee Israel, the British actor plays her openly gay partner-in-crime Jack Hock, who died from AIDS, and he reveals he drew inspiration for his character from an old colleague, who succumbed to the same disease almost three decades ago.
And Grant found a way to honour his memory with a piece of his onscreen wardrobe.
"The person who most inspired what I did - because there was so little to go on research-wise on Jack Hock - was an actor who I was friends with and worked with, called Ian Charleson, who was in 'Chariots of Fire'. He died of AIDS in 1990 at the age of 40," Grant told Entertainment Weekly.
"The bandana I wore in the film was a request from me to (director) Marielle Heller and the costume designer to say, 'This is what Ian looked like the last time I saw him'. That was as much a literal homage to our friendship that I could get into the movie."
"He had this amazing, loose, scalawag (behaving badly) quality as well as a little boy lost, so all of those things were inspired by this man who was of a generation that were (sic) wiped out by that plague (AIDS) in the '80s and '90s, when it seemed like a cure was impossible to ever achieve."
Grant, who is straight in real life, admits he's been touched by the overwhelming outpouring of support the film has received from members of the LGBTQ (lesbian, gay, bisexual, transgender, and queer) community since its release.
"The movie was also given the LGBT Movie of the Year award at the Dorian Awards last Saturday, and I was also given Best Supporting Actor," he shared. "If that body of people is voting for this, that is the greatest approbation the performances could possibly have, because it is honouring people and not sentimentalising them in any way. This is who they are, this is their sexuality, no excuses are made and nothing is varnished up. It's not issue-driven, either; this is just who they are, and that's what I love about it."
His performance has also taken his career to new heights, scoring the 61-year-old his first Golden Globe nomination, and his first Oscar nod for Best Supporting Actor, which was announced on Tuesday, January 22.
"Can You Ever Forgive Me?" also earned Oscars recognition for McCarthy in the Best Actress category, and for screenwriters Nicole Holofcener and Jeff Whitty for Best Adapted Screenplay.