The modern reworkings of the genre are traceable to a few different factors. For one thing, rewriting the rules is just good storytelling. Upending conventions lets you surprise the audience. You thought garlic was going to ward off the boss vampire? Sorry. You planned to kill him with that little piece of sharpened wood? Good luck. These days, you'll see vampires slapping crosses out of the way more often than shrinking in fear. Variations on the vampire rules also make for some clever plot twists. For example (spoiler alert!), in 30 Days of Night, Josh Hartnett notices that once bitten, victims become vampires right away—but they don't become evil vampires for a few hours. He therefore injects vampire blood into his veins so he can fight them off and save his wife. True Blood also has a smart twist on the myth-busting trope: The vampires started the myths themselves. "If the humans thought they couldn't see us in a mirror," explains vampire Bill, "it was another way for us to prove we weren't vampires." Plus, tweaking the rules is part of the appeal of genre fiction—authors have a template to play with, so every minor variation they make becomes loaded with meaning.
These expository scenes are also common because vampires are so darn chatty. All monster myths vary, after all. Sometimes zombies are fast, sometimes they're slow, and it always seems to take a different tactic to kill them. But zombies can't talk, so they can't haughtily explain to you why they're not like all the other zombies. They just chomp your face. Vampires, on the other hand, are the biggest self-promoters around: They can't stop talking about themselves.
Another factor is changing censorship rules. Believe it or not, vampires were not always sexy (although sexuality was part of the mix as early as Carmilla). The original Dracula film came out in 1931, a year after the Hays Code was put in place. So they shot two versions—one chaste English version for American audiences and one Spanish-language version for distribution in Mexico. The women in the foreign version wear lower-cut dresses. Hot vampires really broke out in the 1950s in the British Hammer horror films and finally made it to the United States once the Hays Code was dropped in the late 1960s, clearing the way for Andy Warhol's take on vampire sex.
Technology also plays a role in vampire transformations. Vampire films got gorier once color film made it clear they were drinking blood, not oil. Shoddy makeup on high-quality film stock sometimes made fangs unconvincing. One director, Mario Bava, decided to scrap them entirely—the vampires in the 1960 flick Black Sunday are, like the Hays Code at that time, toothless. Technology within the films plays a role, too. In recent vampire stories, science is the new magic. In I Am Legend, it's the "vampiris germ" that causes vampirism. ("You see, the bacillus is a facultative saprophyte," we're told, which is supposed to explain why a stake causes a vampire to dissolve into dust.) In Underworld, it's a genetic mutation. And as technology evolves, so do vampire-slaying methods. Blade's garlic-filled bullets are nothing compared with the bullets from Underworld that are filled with—kid you not—daylight.
Other variations are introduced because, well, they're totally sweet. The vampires in 30 Days of Night are more feral than human, with their own creepy language and two rows of shark teeth. Needless to say, they don't leave two dainty dimples in the neck. Buffy the Vampire Slayer, meanwhile, features an elaborate universe of humans, vampires, demons, werewolves, slayers, and "watchers." (Sometimes, they sing.) Underworld creates a deep mythology about a war between werewolves and vampires. In Guillermo Del Toro's Cronos, the vampire isn't human at all—it's a tiny mechanical beetle.
But the biggest reason for all the myth-busting has to do with creating a believable world. It may seem odd to explode the myth about crosses in one scene while positing that vampire blood is a sex drug in the next—neither myth is believable, taken alone. But stomping on old myths heightens the realism. It's a way of acknowledging the silliness of most vampire stories while distancing yours from the rest. We know vampire tales are childish, it says. This one is not. That's why you'll always have a character saying he doesn't believe in vampires—the filmmakers know that's what you're thinking, too. The myth-busting scene is therefore a necessary ritual. By rewriting the rules every time, you ask viewers to invest themselves in this story, not in the last vampire movie they saw.
All genres evolve, and in this respect vampire films are nothing special. But vampires seem to relish deviating from their conventions more than most. At the very least, it keeps the genre fresh for Lesbian Kung-Fu Robot Vampire Killers From Space.