Switch coaching styles regularly and discuss your candidate’s reaction to those switches.
Encourage your candidate to practise situations he finds difficult to handle or avoids, for instance in a role play.
Set certain difficulties for your candidate in a role play (“this cannot happen; you cannot do this; this does not work like this”) and encourage your candidate to deal with these problems strategically.
If your candidate has a managing position, bring up situational leadership. All kinds of interviews are good moments to practise these skills, especially when resistance is expected.
Encourage your candidate to ask others for 360º feedback regarding his flexibility. What are his strengths and what could be improved? Discuss the results with the candidate.