Dietz is a minor villain in the novels The Land That Time Forgot and Out of Time's Abyss. In the film, he is played by Anthony Ainley and is the main antagonist.
Dietz was a burly Prussian crew member aboard the U-33. He was one of Lieutenant von Schoenvorts' NCOs and served as an overseer, bullying and beating the British prisoners in Caprona. When John Bradley and Co-Tan rescued the captives, Dietz was killed in the ensuing battle.
Dietz has a much bigger role in the film. Instead of merely being a member of the crew, he is Captain von Schoenvorts' first mate. He is still a brutish bully and would-be war criminal. When the Germans sink the British cruise liner, Dietz requests permission to surface and kill the survivors, but von Schoenvorts refuses because they were civilians.
After the takeover by the survivors from the British ship, Dietz pretends to cooperate with Bowen Tyler and the others, but secretly undermines them. He breaks the radio, sabotages the compass and even frees the imprisoned von Schoenvorts so the Germans can attempt to retake the sub.
In Caprona, he begins growing tiring of his superior's cooperation with their enemies, and even picks a fight with British crew member Olson. One day, Bowen Tyler, Lisa Clayton and some of the British crew go hunting. While they're gone, a volcanic eruption begins, and although von Schoenvorts wants to wait for the hunting party to return, Dietz begs to return to the sub. He eventually convinces the Captain that they'll wait for their comrades at the sub, but, once aboard, he pulls a gun, shooting Olson and von Schoenvorts, and forcing the crew at gunpoint to dive despite the fact that the water is boiling hot from the volcano.
He sees Bowen and Lisa onshore and taunts them with being left behind, before going below. Just before the sub sinks and explodes, Bradley manages to get a gun and shoot Dietz, killing him.