A friend told me of some very valuable recent advice she was given for handling her prickly daughter. The suggestion was that if she needed to ask questions and the conversation was tense, set the context first and then ask the question.
For example: 'It's a big and complicated assignment. How long do you think it will take you?'
She says it works very well and I have successfully experimented with it in other prickly conversations, even when the person isn't a daughter!
The technique fits with the NLP approach of creating a bridge into a difficult question. I guess that the context gives the person time to adjust to the question topic before they have to answer. Presumably the prickly person may feel less defensive as a result?