Letting Your Characters Go

When asked at a talk what was the best way to get over writers' block, actor and author Peter Mark Richman said - to paraphrase -.
"Take two very different characters and put them in a situation, any situation, but make it odd for at least one of them and then just let them talk."
It is good advice for getting over writers block but it is also an odd idea. The concept that your characters can almost act on their own will. This might seem a bit strange and of course the ideas are still coming out of the writers' own heads, but there does seem to be a moment of time when your created characters do take on a life of their own. 
For instance, I put three characters in a semi-abandoned maintenance bay expecting only conversation and perhaps a tense conflict between a pair of siblings that would result in a firm scolding.
Then out of nowhere one of them rips out a chunk of the others hair, MacGivers  a thermite bomb, and burns the maintenance bay to the ground. 
At no time in the three year course of creating or writing the story did this author intend for this to happen but when these characters were put in this situation this was the single most logical outcome. 
​Richman was onto something.