It would be wonderful, albeit boring, if life proceeded smoothly all the time. On the day of the interview, you intend to be well prepared, rested, up early, and in good form. What if you spill breakfast in your lap, jump a red light on the journey, and arrive late to find your manager waiting for you? A bad start to the day! All is not lost; you can learn to switch to "performance mode".
Have you observed how athletes take a minute to get into the zone before performing? Here is a mental trick I use when I have to perform and don't feel like it:
The next step is to prepare the candidate, to relax them so they are ready to be interviewed.