A priori pas grand chose si on en croit ce qu'en dit Peter Jackson dans une interview accordée au site io9.com.
Il a jugé impossible de réutiliser les 18 mois de travaux préparatoires réalisés par Guillermo Del Toro. Ils reflétaient trop la vision personnelle du réalisateur mexicain. Jackson pense que seul Del Toro peut faire du Del Toro et qu'il ne lui était pas possible de se mettre à sa place en se réappropriant son travail. Jackson est donc reparti de zéro. Il a toutefois conservé quelques idées qu'il trouvait très bonnes mais il ne peut pas en dire plus puisqu'elles sont sous le coup de NDA (Non-disclosure agreement ou Accord de non divulgation en français) :

(Guillermo) had designed a lot of the movie . . . I looked at his designs when he took over and a lot of his designs are very Guillermo . . . it was very much stuff that you would recognize from Pan's Labyrinth or Hellboy. It was his artistic vision and I couldn't make that movie. I looked at his designs and I said the only person who can make a Guillermo Del Toro movie is Guillermo. It shouldn't be me. I can't put my head into somebody else's idea — I have to generate it from the beginning. So really I redesigned the film pretty much. Some of Guillermo's DNA is in there — there were some things he did that I thought were pretty cool and I've taken bits of pieces of his stuff — kind of altering it and changing it as I saw it. But the film was largely redesigned.

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