Nathan Lane (Teddy Dimas) uses American Sign Language (ASL) in most of his "Only Murders in the Building" scenes with James Caverly, who is deaf in real life. 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 talented, accomplished musician.
The most-watched comedy premiere in Hulu history.
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.