The final film was written by Jeffrey Boam , but several famous "drafts" by other writers helped shape the story:
And who's gonna come to save you, JUNIOR? Indiana Jones: DON'T call me Junior! Quotes - Indiana Jones and the Last Crusade (1989) - IMDb Indiana Jones y La Ultima Cruzada
This version introduced the Holy Grail as the central artifact and established the father-son dynamic. However, it was considered too serious and lacked the humor of the finished movie. The final film was written by Jeffrey Boam
Though uncredited, playwright Tom Stoppard did a final "draft" to refine the dialogue, particularly the witty exchanges and the emotional beats during the Grail trials. Iconic Quotes from the Script Indiana Jones y La Ultima Cruzada