When engineering vocals, someone must take charge. Is it going to be you? If there is not a specific producer, often the players will fall back on you and hope you can use your experience to guide them.
So do it. Take charge and lead the session. This is where you and the singer get, ahem, intimate. Show confidence, and tell the singer exactly what you want. This is your gig. Do whatever it takes to get the singer relaxed and ready to sing. When the singer has confidence in you, he can relax and do his job.