A diary is usually thought of as sacrosanct-meaning you should never read one unless you wrote it (or it's been published, like Anne Frank's or Bridget Jones'). That's especially true of these movie diaries, which hold secrets more dramatic and dangerous than your average diary – read (or watch) at your own peril.
Humbert's Diary, "Lolita"
Humbert Humbert is a respected academic professor in his community. But if you take a look at his diary, you'll see that old Humbert has more on his mind than books and authors. Namely, 14-year-old Charlotte Haze, or Lolita, whom he wants to do awful, awful things to. When Charlotte's mother sees it, she ends up getting hit by a car, so you'd better keep it closed. Pervert.
Travis' Diary, "Taxi Driver"
On the other end of the respected-by-the-community spectrum is the hero of "Taxi Driver", Travis Bickle. Travis is an insomniac taxi driver with a penchant for really violent thoughts, and his unhinged diary lays his soul bare. As a writer, though, Travis isn't the specific type, so it's unlikely reading his diary would have prevented his terrible crimes.
The Diaries of Angier and Borden, "The Prestige"
Chris Nolan's "The Prestige" is a structurally complicated film, with much of the narrative taking place within one diary and some in another. It's all about ten times more confusing than your average magic trick, so you might be better off just watching the movie and leaving the diary-reading to the professionals.
Harker's Diary, "Dracula"
In many of the versions of the "Dracula" story, poor old Jonathan Harker keeps a journal while visiting the notorious Count. In all versions of the "Dracula" story, it goes badly after that. Do not read unless you want the Prince of Darkness coming after you as well. He is probably not a fan of loose ends.
The Diary in "Eastern Promises"
The underage prostitute's diary that the entire plot of "Eastern Promises" is predicated on is full of dangerous secrets about the Russian mafia. As you may or may not be aware, the Russian mafia are a little bit secretive about their day-to-day operations, so unless you want to end up a corpse with your fingers clipped off and your face smashed in to avoid identification, consider leaving this one unread.
The Diary in "The Cabin in the Woods"
The characters in "The Cabin in the Woods" are pretty much doomed from the start-this is a horror movie, after all. And yet, maybe things would have gone a little differently if they hadn't read that damn diary. Well, no, things wouldn't have been better, but at least the characters would have been slaughtered without having to read the phrase "husband's bulge" out loud.