In the original Knight Rider series KITT was a 1982 Pontiac Trans AM the car was so popular that people went to General Motors wanting to buy a Pontiac like KITT General Motors got so pissed off that they asked the makers of the show to stop referring to it as a Pontiac so from then on people would refer to car as being a Black T Top.
Actor Rick Cosnett his played Eddie Thawne was originally cast as Jay Garrick in season one of The Flash.
When filming the scene in T2 Judgement Day when the T-1000 runs after John Connor who is riding his motorbike out of the car park,James Cameron had to tell Edward Furlong's stunt double to ride faster as Robert Patrick had got so physically fit,he kept catching up with the bike
A couple of Harry Potter ones.
Adrian Rawlins,who played James Potter,Harry's father,had played Arthur Kidd the leading male character in the 1989 TV version of The Woman In Black. Following the end of the Potter films,Daniel Radcliffe played Arthur Kipps in the film version of The Woman In Black.
In the Prisoner Of Azkaban,when Harry gets off the Knight Bus and enters the public house The Leaky Cauldron,the patron sitting at a table reading a book is Stone Roses lead singer,Ian Brown,in an uncredited role.
Director John Carpenter originally wanted Christopher Lee to play Dr Loomis in Halloween.Lee turned it down and the role eventually went to Donald Pleasence.A few years later,Lee bumped into Carpenter at a studio party in Hollywood and told him that turning down Halloween was the biggest mistake of his career.
Tom Cavanagh and Matt Letscher had to share the same Reverse Flash suit baring in mind that Tom Cavanagh has a much smaller frame then Matt Letscher it's only until season three of The Flash that Reverse Flash was then specially made for Letscher.
Here are some interesting facts CW actors who have played other DC characters:
Wentworth Miller who plays Captain Cold also voiced Deathstroke in the animated series Young Justice.
John Wesley Shipp who played both Henry Allen and Jay Garrick also played Barry Allen in the 90's Flash series he also voiced Reverse Flash in the Batman animated series The Brave And The Bold.
Neal McDonough who plays Damien Darhk has voiced Green Arrow in DC showcase animated shorts and Batman/Shazam animated, Deadshot in Batman Assault On Arkham Animated movie and he voiced both The Flash/Barry Allen and Nightwing/Dick Grayson in the video game Injustice: Gods Among Us, so his been very busy lol.
Disney rejected Back To The Future because they thought the film was too dirty with Marty's mother falling in love with him lol.
The famous Kong vs T Rex(or meat eater as it was simply called)scene in King Kong(1933)was originally just test screen footage that Willis O'Brien animated to see how good it looked.Director Merian C.Cooper was so impressed,he simply incorporated it into the final cut.
Carrie Fisher was a script doctor and would polish up scripts she even rewrote some of dialogue for the original Star Wars films and even polished up scripts for the prequel Star Wars films too.