To untangle these credits, there are a number of points to you need to understand:
When two names are separated by an ampersand (&), it designates a writing team. So Ashley Edward Miller & Zack Stentz are two writers who work as a team.
When two names are separated by "and" that means that the writers did not collaborate as a team, but rather contributed to the script at different times during development. Perhaps one rewrote an earlier draft by the other.
The order of names indicates the weight of the writer's contribution to the final shooting script. The name that comes first contributed more.
Sometimescredit is splitinto "story by" and "screenplay by." The WGA defines "story" as a contribution "distinct from screenplay and consisting of basic narrative, idea, theme or outline indicating character development and action."
On a script (like Thor) that has been in development for many years or even decades, dozens of different WGA writers may be hired to work on the script at different times, but only two writers (or writing teams) can get credit. In the case of Thor, a writing team and a single writer split screenplay credit, and two writers split story credit.
When a screenplay is non-original (based on underlying material, as Thor was based on a comic book), a credited writer must have contributed 33% or more to the final shooting script. For "screenplay by" credit the writer must contribute 33% in the areas of character, structure, dialogue, and "new and original scenes."
When a screenplay is original, the first writer gets sole credit, unless a subsequent writer has contributed 50% or more to the final shooting script.
When a credit reads "written by," it means that the writer(s) is responsible for both story and screenplay. For example, on Conan The Barbarian 3D, the credits read: written by Thomas Dean Donnelly & Joshua Oppenheimer and Sean Hood. So, the writing team Donnelly & Oppenheimer share both story and screenplay credit with Sean Hood (me.)
In the case of Thor in particular, I know that Mark Protosevich was hired by Marvel Studios to write the script in 2006, and he did another draft in 2008. However, when Kenneth Branagh was hired to direct it, other writers were hired to do rewrites. Also Thor had been "in development" since 1990, so its possible that some of the writers who worked on scripts before Protosevich were eligible for credit. Since Protosevich did not get "screenplay by" credit it seems that only some general elements of "story" remained in the final shooting scripts after many rounds of rewrites. But who is really responsible for what? Having been through several WGA credit arbitrations (including one for Conan The Barbarian 3D) I can say that each writer may have very, very different ideas about who deserved credit for what, and who "really" wrote the script. Check out this New Yorker article on credit battles. Also, check out this story: George Clooney was furious when the WGA decided not to award him credit on Leatherheads, a script that he claims he completely rewrote: WGA, Clooney at odds over credit
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Screenwriters are filmmakers. So, in the same way that aspiring architects study buildings, aspiring screenwriters should study, first and foremost, the films themselves. A book like Screenwriting: The Sequence Approach, by Paul Gulino, has beat by beat analysis of well known movies that illustrate story structure, sequencing, plant and payoff and other techniques.
That said, there are a number of scripts that beautifully illustrate how screenwriters use the written word to indicate visuals, tone, style, pacing, structure, character, tension and action: words that define the finished movie like the lines of an architect's blueprint.
It is also very instructive to look at early drafts of screenplays to discover how the film was changed and revised during production. Sometimes brilliant scripts were made into mediocre movies. Sometimes problematic scripts were solved in the process of shooting and editing.
Here is a list of feature scripts that agents, executives, film school professors, and screenwriters themselves often cite as influential and instructive to aspiring screenwriters.
Five mainstream Hollywood scripts often cited as perfect in style, structure, content, and execution are:
Since so many movies are adaptations, it is instructive for screenwriters to read the underlying material (usually a novel), and then read the screenplay adaptation. Five scripts often recommended are:
Cider House Rules
Silence of The Lambs
The Shawshank Redemption
The English Patient
Classic Scripts most often recommended:
Treasure of The Sierra Madre
The Sweet Smell of Success
Some Like It Hot
His Girl Friday
Taste in indie film is more eccentric and subjective. But I would list these independent voices as instructive to ALL screenwriters:
Ronnie Rocket (unproduced script by David Lynch
The Spanish Prisoner
There are certain celebrity screenwriters who are able to write in such a way that reading the script is an enjoyable and enlightening experience, separate from the experience of watching the film. Love them or hate them, it’s instructive to read scripts by:
Paul Shrader William Goldman Charlie Kaufman Shane Black The Coen Brothers
Female scribes are often neglected in these lists, but men and women alike can learn from the unique voices of:
Norah Ephron Sophia Copola Callie Khouri Allison Anders Diablo Cody
The key is to look for EARLY DRAFTS OF THE SCREENPLAY, not just the official “shooting script” which is often a simple transcription of the finished film.
Even maverick writer-directors working outside the Hollywood system and writers who want to challenge mainstream formulas of three-act-structure, conflict-centered storytelling, and concept-driven subject matter would do well to study these scripts. A screenplay is ultimately a communication tool, like a blueprint, to be given to actors, cinematographers, production designers and editors. Even if you aspire to make movies like Tarkovsky and HATED Lethal Weapon, you can still learn from the way successful screenwriters clearly and competently convey style and cinematic action to their collaborators.
As a final note, perhaps the best place to start is to read the screenplays for a couple of your favorite films. Reading them will feel like play rather than work. And there is no faster way to learn the format and tools of the screenwriter's craft.