Apparently, the WGA is very protective when it comes to awarding writing credits to so-called production executives (directors, producers and EPs). I guess this prevents the director/producer from easily claiming credit for brainstorming on the story or punching up some dialogue, etc. These efforts are considered a typical part of the director/producer job description and dont warrant an extra writing credit. Accordingly, the WGA imposes a strict criterion: the production executive has to contribute at least 50% original content (however thats measured
) - whereas, (e.g.) the original writer may only have to contribute 25 or 33% in order to get official credit.
It seems that Jenkins and Johns extensively rewrote drafts - such that WB
believed they had passed the 50% test and were entitled to a co-screenplay credit. But (presumably) the WGA thought otherwise.