Mary Shelley, Peggy Webling, John L. Balderston, Francis Edward Faragoh, Garrett Fort
It takes a lot of people to bring a dead corpse to life. In Frankenstein, you've got Frankenstein himself, and then his assistant Fritz…but you've also got whoever donated that nasty brain, and all the people who used to be using the dead arms and legs and other bits. Frankenstein's monster isn't just Frankenstein's; lots of other people (and corpses) pitched in.
Writing Frankenstein took almost as many hands—many of them dead hands, as it happens. The script for the film was inspired by Mary Shelley's 1823 novel, about a scientist who brings a monster to life.
But Shelley's monster reads Milto and argues philosophy and is generally not much like the grunting dude you see in the movie.
The 1931 film was based more directly on a 1927 play by Peggy Webling—Webling threw in a servant for Frankenstein (the mean and cranky Fritz), and also dumbed the creature down. No Milton for you, monster. (Source)
John L. Balderston, who had done rewrites on Webling's play Dracula for the Universal pictures version, was called in to tinker with and restitch Frankenstein for the film. He was joined by veteran screenwriters Francis Edward Farragoh and Garrett Fort. Together, they experimented and eventually came up with a script more sympathetic to the monster than Webling's play. (For instance, in the play, Fritz accidentally scares the monster with fire; in the play Fritz deliberately torments the creature.)
In the play, and in the initial version of the film, Frankenstein dies with the monster in the windmill…and then Victor ends up with Frankenstein's fiancée Elizabeth. Little cut up bits of this plotline survive—but test audiences hated that. It wasn't happy enough for Hollywood.
So the writers huddled and came up with a different ending, where Henry Frankenstein survives. Given how much of an unstable dope Frankenstein is, this maybe isn't actually such a happy ending for Elizabeth…but Hollywood is Hollywood. (Source)
So good job Mary Shelley, Peggy Webling, John L. Balderston, Francis Edward Faragoh, and Garrett Fort—it took a lot of you, but altogether you turned a mad scientist into a happy romantic lead.
It's aliiiiive! It's getting maaaaarried!