What would be your motivations for a change?
What elements would you consider for a change? In what order of importance?
What are your disappointments in your current job and what would you like to change for your next job?
In the case of a response to an advertisement, find out if the candidate is on an intensive search or answered the advertisement just because they are interested in something in particular.
In how many searches have you participated? For what positions? What expectations do you have on each of them?
Have you ever had a counteroffer when you presented your resignation letter?
What motivated you to change when you left
?