The 30-year-old actress stars alongside Mackenzie Davis in the new film, which follows a young woman who wants to propose to her girlfriend at a family holiday party - only to discover her partner hasn't come out to her conservative mum and dad.
Kristen, who has been in a string of same-sex relationships since splitting from her "Twilight" co-star Robert Pattinson in 2013, says that growing up would have been much easier if more big, family-friendly films addressed LGBTQ+ relationships.
"I didn't grow up with a movie that had such expansive ambition, that had two female leads at the centre of a love story, and not in this format," she tells British newspaper The Guardian. "Not to say there hasn't been great queer content made over the years that has been really beautiful, and really important touchstones. But at the same time, it's not something that I grew up with, and I would have loved to. So it feels good to be a part of it."
The movie is directed by actress and filmmaker Clea DuVall, who identifies as a lesbian, and Kristen sees "Happiest Season" as a real sign of progress for LGBTQ+ representation in Hollywood.
"I think it's just evidence of progress and desire," she adds. "You know, it's one foot in front of the other. It's pretty obvious to anyone that's ever engaged with this thought process that the answer would be: because we're choking for it, obviously, and so there needs to be more."
"Happiest Season" is available to buy and rent from 26 November.