The contextual interview goes through multiple iterations of a 2 step pattern:

• Watch and learn.
• Stop and initiate discussion when the user does something you don’t immediately understand, when you want to confirm an interpretation, or to allow the participant to validate or invalidate your understanding of the user’s mental model

Try to understand the underlying processes:

• Be aware of external resources being used.
• Ask about standard steps vs. extraneous or uncommon variations in their processes and the reasons behind them.
• Explain your interpretations of their tasks and workflow for the users to confirm or correct.