You should never let your guard down. I dare say that a few good instructors may have been killed by having overconfidence in there student or pilot being checked out.
I was given the task to sit with a new instructor some years ago. He needed to get 5 hours on type, was short of 2, so was good opportunity for him to get to know the local area. Seemed quite confident and i made the mistake that i could just relax and enjoy the flight.
First landing on roundout the world disappeared and we were heading for a stall. I just sat there waiting for the go around.
Which didn't happen. I grabbed the controls and applied full power. I was shocked that such an experienced pilot could freeze at the controls.
....and seen the result of two very experienced instructors total a twin between them.