Why did the doctor sit like a stone?

This question is from the story 'The Snake and the Mirror'. The doctor was absolutely petrified, mate. A massive cobra had dropped from the rafters onto his shoulder and coiled itself around his arm. He sat like a stone out of sheer terror. He knew any sudden movement, even a deep breath, could provoke a fatal bite from the snake. His stillness was his only chance of survival, a desperate attempt to not appear as a threat.