Reg Adkins at Elemental Truths has written an article on ways to perform verbal intervention for de-escalating a volatile person. He mentioned couple of tips: Paraverbals, Reflexive Questioning, Silent Treatment, Calm as Lake, Keep it Simple, Body Language and Divide and Conquer:
We hope we will be able to conduct operations in such a manner that we will never be called on to use intervention techniques designed for de-escalating volatile persons, but occasionally such techniques become necessary.
7 Keys to Effective Verbal Interventions – [Elemental Truths]