One of the things I've learned about students over the years is that most of them have very good hearing and vision. Students often seem to best hear the throwaway lines we didn't really mean. What they most often see is what we didn't mean to do. We need to remember that everything affects the way students think about aviation.
During one flight review, I recall a fellow who confessed that he would sometimes just hop in the airplane and go, without doing even a cursory preflight. I'll admit that there are different degrees of preflight intensity. The one you do after any maintenance is performed on your airplane is certainly a bit different from the one you do if you had flown the airplane earlier that day.
It's reasonable to expect someone to always at least check the essentials (fuel, oil) before departure. After just a bit of questioning, it became apparent that this person's flight instructor skipped the preflight himself on occasion. It's not a great leap to see where the student learned it's OK to pick and choose your preflights.
The things we say and do, however casually, can have an enormous influence on our students' behavior. Suppose your student overheard you saying, "It's not always necessary to check the weather before a flight," or "Sometimes it's OK to skip the flight planning and just go flying." Granted, there are times when it's not necessary to do these things, but unless you explain why in detail, all your student remembers is the "It's not necessary" statement, out of context.
When students are within range, we should assume everything said or done is permanently recorded--because it is.
By Rod Machado