Nathan Lane (Teddy Dimas) uses American Sign Language (ASL) in most of his Only Murders in the Building scenes with James Caverly, who in real life is deaf. Lane had only six weeks to learn enough ASL for his emotional scenes with his on-screen son in "The Boy from 6B"
Steve Martin is actually playing the concertina in various scenes. He is in fact a very accomplished and talented musician.
A running gag on the series is that podcasting superstar Cinda Canning (Tina Fey) surrounds herself with similar-looking assistants, some of whom also have similar names. One of these assistants, Cindy, is played by Anne Stringfield, Steve Martin's wife, whose resemblance to Tina Fey has been fodder for media comments since Stringfield and Martin married in 2007. Fey herself has often joked about this; for example, while helping to present Steve Martin's AFI Lifetime Achievement Award in 2015, Fey quipped, "I've learned so much from you, Steve, and I can't tell you how grateful I am to know you. You have helped me so many times and in so many ways: showing up for SNL monologues; being a guest star on our dumb TV show; marrying a woman who is the younger, thinner, smarter version of me, just to keep me hustling." The role of Cindy is Stringfield's first credited professional acting job.
In August 2022, Steve Martin revealed in an interview that this would likely be his final acting role, as he did not intend to seek out additional roles or cameo appearances once production on the series concluded.
A June 2022 New York Times article by Penelope Green recounted the history of the Belnord, the apartment building on the Upper West Side of Manhattan (at West 86th Street and Broadway) that stands in for the "Arconia" building in this show. Green wrote that when the Belnord was built in 1909, it was publicized as "the largest apartment building in the country, and maybe the world," and it remained a prestigious place to live into the 1960s (celebrities with apartments there included Lee Strasberg, Walter Matthau, Zero Mostel, and Isaac Bashevis Singer), but its owner became "both litigious and recalcitrant, refusing to fix even the simplest issues, but energetic enough to sue not only her tenants but also the landlord association that threw her out for not paying her dues." This led to a sixteen-year rent strike and many lawsuits; finally the building was sold in 1994 and its new owner put over 100 million dollars into renovations until it was restored to its former luxury.