A rock can be an Antagonist, if the Author attributes some reactive responsibility to the inanimate object. They need to Prevent. And they need to Reconsider. says Jim Hull
From what I've read in the dramatica theory book. Every archetypal character has to characteristics, an action characteristic and a decision characteristic.
When creating complex characters I believe the action and decision characteristic can be assigned to different players. Meaning, the rock can play the action characteristic of prevent, while an assistant to the captain on the ship maybe suggest that the crew reconsider sailing in the turbulent waters because the rock could damage the ship. In other words the member of the crew speaks for the rock when it comes to playing the role of reconsider.
The rock prevents
Unless someone represents the rock or speaks for the rock, it won’t come across as a character,
A member of the Persian crew named Darius urges the other characters to reconsider
I don't mean to negate what you've said @jhull. I just needed more clarity on what you mentioned in your blog post.
In addition, the rock may not the role of prevent for the entire narrative but only for that dramatic circuit/unit or scene and then that role or prevent could be handed over to an the crew of an enemy ship for example a Greek ship.
After the handoff, the crew of the Greek ship could start hurling cannons at the Persian ship that was trying to navigate between the rocks playing the role of preventing the Persian ship from reaching it's destination.
Darius who urged the Persian crew to reconsider is a double agent and he too begins to slaughter the Persian crew when the Greek ship starts firing cannons and so Darius would play the role of prevent too once the Greeks start firing.