To promote their new film, the actors took to the Broadway stage, and the cast turned their cheeky banter into an impromptu love song. “We are so outside of our comfort zones,” Helen Mirren was saying as she sat with Ian McKellen onstage at the Booth Theater on Sunday night. The two are no strangers to the New York stage; Mirren, 74, and McKellen, 80, played opposite each other in a Broadway production of “Dance of Death” in 2001. But this time, they were the surprise guests at “Freestyle Love Supreme,” the improvisational hip-hop comedy show that was co-created by Lin-Manuel Miranda. The esteemed British actors, co-stars in the forthcoming film “The Good Liar,” didn’t rap; instead, they were interviewed by the show’s M.C., Anthony Veneziale, about their personal and professional camaraderie, and the result was turned into an impromptu love song by the regular cast onstage. The fact that “Helen” and “McKellen” rhyme was not lost on the rappers. A complete bit on the evening can be read over at The New York Times, while pictures have been added to the photo gallery.