It is accepted that a Hollywood studio with money to spend needs a known commodity to base a movie on. The truth is that studios are unwilling to gamble on projects without name recognition, and prefer to adapt almost any popular item into - hopefully - a popular film.
Yet with comic-book and superhero films having their reboots rebooted (another Spider-Man film, anyone?) and every conceivable brand name from Battleship to Fifty Shades of Grey already acquired and adapted, the well of well-known is running dry.
Hollywood's latest solution is taking old tales - even true stories - in one hand, and previously discreet, defined genres in the other, then rubbing them together. Hey presto: popular stories with a twist. This super-genre has become known as ''mash-ups''. But one of the genre's high priests isn't so happy with the term.
"I understand why it's the easy interpretation," says Seth Grahame-Smith, who wrote the novel and screenplay Abraham Lincoln: Vampire Hunter, "but it evokes the image of me standing throwing darts at a wall with different words and names and concepts on them and then just deciding 'oh, those two'. That's not at all how I approach it."
Grahame-Smith prefers ''Absurd Literature'' or ''Absurd Premise Fiction'' to describe his work, having achieved bestseller status with his Jane Austen variant Pride and Prejudice and Zombies. He's no Dr Frankenstein for this genre monster, he says. Instead, his novels stand on the shoulders of some well-known giants.
"They're no different than what Joss Whedon did when he wrote Buffy the Vampire Slayer. It's no different to what Gregory Maguire did when he wrote Wicked. It's just taking well-worn territory and looking at it from a slightly shifted perspective, getting a different way into a familiar story."
He sees Douglas Adams's Hitchhiker's Guide to the Galaxy series as another example.
"I loved that book growing up. And it didn't occur to me that Adams was deconstructing science fiction or doing anything to me, it was just clever."
Unsurprisingly for a man who has made a beloved American president an action hero, Grahame-Smith pulls no punches in defence of his style. "The worst thing you can do as a screenwriter, as an author, is just to retread well-worn ground, just to become an imitation of yourself,'' he says.
''There are movies - and we all know what movies they are - that from the very beginning of their development process all the way to their release just seem like exercises in cynicism and exercises in toy branding and marketing rather than making something truly entertaining.
"We understand how insane the [Lincoln] concept is. We are doing it unflinchingly and unapologetically."
For Benjamin Walker, who plays Lincoln, the insanity of the concept was (and is) a drawcard: "I read a lot of scripts. And most of the time, there is no 'Really?' It's mostly, 'How did that get money?' or 'This is so stupid, I've seen this 20 other ways' or 'Another one of those? Jesus - kill me!'"
But there is no doubt that taking liberties with a real historical icon is a new step for mash-ups.
For Grahame-Smith, no material is off limits.
"I have sacred cows, in so far as again you have to treat your subjects as sacred. With Lincoln I spent months becoming an amateur Lincoln scholar. Trying to understand the man in a deep way and trying to understand the realities and the suffering of his life in a deep way so that I could transfer them into this ridiculous story."
Far from upsetting history buffs, the book and film have thrilled many Lincoln experts.
"We've been embraced by the Lincoln community, the scholars, the museum and library … because I think what people realised - the people that are the keepers of the Lincoln flame, the protectors of the legend - this is something that is respectful, in a weird way. And it opens up the Lincoln legend to new people."
Plenty won't agree. And it's a fair bet they won't like his next choice of source material: the Bible. Grahame-Smith's next novel, Unholy Night, is described as "the next evolution in dark historical revisionism", which casts the Three Wise Men as thieves fighting monsters and ghosts. "I just had a question that I couldn't answer, an itch that needed scratching … What were those wise men doing there that night?'' Not wanting to ''disrespect anyone's faith'', Jesus is a two-week-old infant in the book, and Joseph and Mary are ''paragons of virtue''.
Grahame-Smith knows which universe he'd most like to mash.
"Star Wars - what kid doesn't want to play in that universe at some point? Of course! No, let's make the Han Solo: Wookiee Hunter movie. Absolutely!"
● Cowboys & Aliens (mix of westerns and sci-fi genres).
● Firefly and Serenity (western and sci-fi).
● Buffy the Vampire Slayer (vampire and high school/coming of age).
● The Hitchhiker's Guide to the Galaxy (sci-fi and absurdist comedy).
● Midnight in Paris (biopics and Woody Allen - yes, he is a genre).
● Desperate Housewives (soap opera, murder mystery and satire).
● Snow White and the Huntsman (fairytale, historical epic, religion).
● Once Upon a Time (fairytale and soap opera).
● Grimm (fairytale and detective).
And next year …
● Hansel and Gretel: Witch Hunters (fairytale and action).
Abraham Lincoln: Vampire Hunter opens on Thursday.