In a February 2010 interview with the UK's 'Daily Telegraph', Clint Eastwood described "Hereafter" as "...three different stories with people who have gone through some sort of stressful time and it's about how they sort of converge together. Much like a lot of French movies have been in the past, where the stories kind of converge together, and destiny drives each person towards the other."
Several biographies describe how Clint Eastwood was stranded offshore in the ocean several days after his Navy training plane crashed. The experience is probably what helped him film the tsunami scene in Hereafter.
One of the few films executive produced by Steven Spielberg that has neither an Amblin Entertainment or DreamWorks Pictures symbol in the "Hereafter" ads representing his involvement. (The Amblin logo does appear at the end of the film itself.)
The restaurant in which Thierry Neuvic and Cécile De France have dinner is Sanderens, the restaurant of chef Alain Sanderens, who became a sensation when he relinquished his 3 stars and reopened the prestigious Lucas Carton under his own name - and with a more affordable menu. Since then, the place has regained 2 stars.
The outside scenes of the London Underground feature Liverpool St Station which is at the eastern edge of the City of London, with the signage digitally altered to make it appear to be Charing Cross. Charing Cross actually has only one above-ground entrance to the tube network, on Trafalgar Square just across from Nelson's Column.
The trivia item below may give away important plot points.
This movie was released in 2010, the same year as a game called Heavy Rain. In the movie a boy called Jason gets hit by a car and dies. In the game a boy with the same name also gets hit by a car and dies.