Attaining the right level of assertion can be challenging.  What one person finds assertive, another may find plain rude.  Two key things to bear in mind here – Firstly, tone of voice is critical.  It’s often not what you say but the way that you say it.  Secondly, if your motivation is genuinely to try to help your candidates and clients with your advice and intervention, this is likely to leak through into your tone and language selection.  If your advice or intervention is motivated by a desire to help only yourself that’ll come through too leading to a far less effective interaction.