Ask your candidate to describe a situation in which he encountered resistance. What did this do to him? What was the other person’s reaction? What went well and what could be better? Try to think of alternative assertive responses together.
Ask your candidate to display behavior that is atypical of him. This can first be done during coaching, later possibly ‘for real’.
Ask your candidate to take careful notice of another person’s verbal signals and body language while talking to him. Explain that the two may diverge - which can be to his advantage. If the candidate’s lack of assertiveness is due to a personality trait such as low self-esteem or an introverted disposition, there is little one can do; in that case this competency is difficult to develop. You can find out with the TMA Assessment.
Confront your candidate with certain difficulties in a role play (“this cannot happen; you cannot do this; this does not work like this”) and encourage him to deal with these problems strategically.