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.
The most watched comedy premiere in Hulu history.