I think he phones the scripts for JL in intentionally. He knows the book will sell anyways, so he concentrates on the books that trully need it, like Aquaman. I cant find another explanation for the utter **** that is JL, when at the same time Johns has given us the amazing GL, Aquaman and Flash runs, as well as the excellent new origin story for Shazam. His strength has always been writing the less popular heroes and making them popular. But i think he has admitted that he doesnt get Batman, WW or the New Gods and that shows.
I think he was involved with the production and writing of the GL movie which makes sense considering he's THE writer of GL. The one who made the franchise a best selling property worthy of a movie. I think they were going with a very nightmarish visual for Oa, the Guardians and the Lanterns, and Johns steered the plot and designs closer to canon. Sinestro actually looked like Sinestro and not like Ming like the concept art suggested. But in the end they mocked it all up, the plot, the CGI, even the casting. Such a pity.
So at the end of the day, when they shoot a character, i think that they should consult with the best writers of said character, but the final decisions should be made by the director who knows the medium of cinema best. But they should consult with the writers definitely. Can you imagine them talking with Morrison instead of just carrying around his book? I can picture him slapping Goyer across the face and going: "NO! NO! Just no!"