The article summarizes the script in detail:
Of the many differences between Dune Messiah in novel form and David Lynch's script, the biggest lay in the opening pages, which detail what happens in the aftermath of the scene in the first Dune movie when the Harkonnens bombed the Atreides’ fortress in Arrakeen, the capitol of the desert planet Arrakis. In the hallway where Duncan Idaho (Richard Jordan) was shot in the head, his shielded dead body still floats on the floor, humming and sparking.
From out of the shadows emerges a familiar face: the Baron's Doctor (Leonardo Cimino). Thought to be the only speaking part created specifically for Dune by Lynch, we learn this Doctor was actually Scytale, a shape-shifting “face dancer” crucial to the plot of Herbert's second book. Going back to Dune ’84, you may not have noticed Cimino's Doctor accompanied Baron Harkonnen during the Arrakeen attack. The Doc is absent after that, even as the Baron yells creepily, “Where's my doctor?” That’s because Doc/Scytale absconded with Duncan’s body. This Easter egg is Lynchian world-building at its best.