This week marks the 25th anniversary of Tim Burton’s Batman, a movie which, back in 1989 broke box office records and reinvigorated our obsession with the caped crusader. But before the film’s release, star Michael Keaton nearly managed to ruin it for everyone, when he revealed a little too much during an late night appearance on 'Letterman'.

Michael KeatonNo spoilers please Michael Keaton

The day before Batman was set to open in theatres, Michael Keaton was out on the promotional trail and stopped by Letterman to help hype up the movie. Not that he really needed to, Warner Bros were already making sure that Batman was the one movie on everybody’s mind that summer. The studio had launched a massive marketing campaign that boasted a whole range of Batman related merchandise, as Keaton happily discussed with the chat show host.

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But aside from Keaton and all the merch, the other big draw of Batman, was of course Jack Nicholson, who would play the caped crusader’s arch nemesis, The Joker. Keaton was happy to discuss with Letterman the joys of getting to work his idol, whom he described as being “one of a kind.” But when the chat show host began asking about the movie’s plot, his enquires about Nicholson’s character caused Keaton to make an unthinkable error.

Letterman decided to ask Keaton just what Nicholson’s Joker could have done to “irritate” Bruce Wayne and make the pair enemies. The actor began to answer by first saying that Bruce Wayne had witnessed the murder of his parents when he was a kid. But before he could go any further, Letterman quickly interrupted him, saying, “and The Joker had done that?” Then, seemingly without thinking, Keaton answers “yes but he doesn't know it till later.” The actor quickly realised he might have given a little too much away and then said, “I’m kind of blowing the plot here.” As the audience gasp and a likely nervous Keaton shuffles about in his chair, Letterman jokes, “like nobody’s going to go see it now.”

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Of course people did still go see it and eventually the movie would gross over $411 million at the worldwide box office. But for those who had seen Keaton’s Letterman appearance, they were left a little less shocked at the film’s plot twist.

Michael KeatonMichael Keaton, happy to spoil his own movies

In the days before YouTube, social media and the term ‘spoiler alert’ existed, Keaton’s slip of the tongue probably only meant something to those who happened to be watching Letterman at the time. For everyone else, the movie probably remained unspoiled (unless you had a particularly thoughtless friend who decided to tell you anyway). Still the video represents a different time in Hollywood before the online revolution, when finding out a movie’s plot twist was actually shocking. Now all it takes is accidentally reading the wrong tweet or status update and you’ve found yourself falling victim to a spoiler. But it seems back in 1989, loose lipped Michael Keaton was all you had to worry about.