- Actor Allusion:
- Abe puts a much younger photo of himself on his online dating profile, which is of course actually a publicity photo of Judd Hirsch from early in his career.
- In one episode, Henry lovingly describes a Jaguar to the detectives. At their weird looks, he explains that he has a thing for Jaguars. Ioan Gruffudd owns a black Jaguar XK8 and loves driving it around LA.
- Henry is seen dyeing his temples silver to try and fake that he's aging and ends up looking like Ioan Gruffudd did in a couple of films.
- Adam is revealed to be played by Burn Gorman; this isn't his first time dealing with the immortal.
- Of course Abe would know what type of cap cab drivers would wear.
- In a flashback to the 1980s, Abe is played by Hirsch's NUMB3RS co-star David Krumholtz, who played his youngest son in said series.
- Isaac states in his first scene that he hates public speaking, but is still good at it. Cuba Gooding Jr.. is well known for his Oscar acceptance speech where he managed to make the "playing off" music sound like the backdrop to a Rousing Speech.
- Henry abhors slavery and tried to help some of the slaves to escape. In Amazing Grace, Ioan Gruffudd played William Wilberforce, one of the main campaigners to end slavery in the British Empire in roughly the same time period.
- Actor-Shared Background: Donnie Keshawarz was in a rock band before becoming an actor, so the writers worked this into Hanson's background in "Punk Is Dead."
- Fake Nationality:
- Abigail Morgan is played by American actress Mackenzie Mauzy.
- The Frenchman is this in universe (she's actually Japanese, not French) and is played by a Chinese-American actress.
- The Other Darrin: Abigail is portrayed by MacKenzie Mauzy in flashbacks from 1945 to the 1960s. Janet Zarish plays Abigail from the 1980s. They've appeared in the same episode, albeit in different flashbacks.
- Playing Gertrude: MacKenzie Mauzy is 26 and has played Abigail in flashbacks from 1945 to the 1960s. Normally this isn't too much of an issue, but flashbacks to 1965 has Abigail interact with an 18-year-old Abe while Mauzy plays the role without any noticeable old age makeup.
- Playing Hamlet: An older version than usual, as Abe is explicitly stated to be 69 years old, whereas Judd Hirsch is 79. He looks good enough for his age to make it plausible.
- What Could Have Been: During a Twitter Q&A in January 2016, Matt Miller explained some ideas that were considered for Season Two, which had been mapped out prior to the series' cancellation:
- There would have been an immortal who cut off Adam's life support in order to resurrect and then team up with him.
- Henry would have had a fling with a younger immortal (although it is unknown if this would have been the same immortal who resurrects Adam).
- Through flashbacks, we would have met another one of Henry's past loves, a "Latin lover". We would have also met Henry's child with this woman.
- Lucas would have been the next character to learn of Henry's immortality.
- There would have been flashbacks to Henry's childhood, Abe learning to ride a bike, and Abe learning of Henry's immortality.
- Lt. Reece would have had more of an expanded role.
- There would have been more about Jo's past, including a story about her criminal father being the reason she became a cop.
- There would have been a "wink-wink" hint that Henry inspired Sir Arthur Conan Doyle to write Sherlock Holmes.
Trivia / Forever