Series creator Matt Miller got the idea for Forever from a conversation with his five-year-old son about death, which led him to imagine a person who was immortal while everyone else, including their own children, was mortal.

Miller intentionally designed the immortal protagonist to have adopted a mortal son who would appear decades older than him, a family dynamic he felt had never been shown on television before.

Judd Hirsch was the first actor cast, taking the role of Abe, and was the only person the producers approached for that part.

Casting director Barbara Fiorentino and Matt Miller struggled to fill the lead role, unsuccessfully auditioning actors from New York, Los Angeles, Canada, London, Australia, and South Africa before Miller spotted Ioan Gruffudd in a preschool carpool lane and thought of him for the part.

The role of Lieutenant Roark was originally written for a male character, but was cast with Barbara Eve Harris in the pilot before being recast as Lieutenant Joanna Reece and played by Lorraine Toussaint from the second episode onward.

Actor Donnie Keshawarz's character Mike Hanson did not have a first name until the fifth episode, when Jo called him 'Mike' on screen; Miller only learned about it afterward when Keshawarz mentioned it to him.

Burn Gorman's immortal antagonist character 'Adam' was only heard as a voice in the pilot and second episode, not appearing on screen physically until the episode 'Skinny Dipper.'

For Henry's rebirth scenes, the crew originally planned to film in the East River but had to shoot in a university swimming pool against a green screen due to the strength of the river's current, later superimposing the pool footage with East River footage to create the illusion.

To depict Henry's deaths across different historical periods, the production team reshot his rebirth scene multiple times with Gruffudd wearing different hairstyles.

The production received a letter from RadicalMedia in 2014 requesting a title change because the novel 'Forever' by Pete Hamill had a similar premise of an immortal man tied to Manhattan; Warner Bros. stated Miller had never read the novel, and Hamill did not pursue legal action.

Despite being cancelled in the U.S., Forever became a major hit in France, drawing an average audience of 5.7 million viewers and becoming the most-watched series on French Tuesday nights.

Fans launched a #SaveForever social media campaign after cancellation, and a Change.org petition gathered over 107,000 signatures by November 2015.