The point you are trying to convey to the passengers is that we do this every day as well, and every day before every take off we brief each other as to the procedures we will likely adopt in the event of various types of emergency. The primary reason for doing this, is that it places those procedures into the front of each others short term memory. This is extremely useful in avoiding the reliance on instinctive behaviour that might otherwise result in high stress situations, when such action might be undesirable.
An example that is often given is the use of seatbelts in the cabin. Who doesn't know to fasten and unfasten a seatbelt? Except that the seatbelt 99% of people are instinctivly used to fastening and unfastening is the one they use everyday, in their car. Guess what, it operates in a different manner. In periods of high stress, without a recent refresher, it has frequently happened that evacuations have been impeded by passengers fumbling to find the buckle, while they look down the side of their seat for it!
There is a balance that has to be struck, between conveying a measured dose of important and correct information, and not putting the fear of God into people unnecessarily. Unfortunetaly this is often a balancing act that is not easy to achieve. Many Captains will use their own authority to also impress the need to to pay attention to what is a routine announcement, because it is so important. A serious emergency is a relatively rare event, but even then, most are perfectly survivable if all the participants are properly briefed, aware, and have their wits about them. No crewmember and no Commander wants to be haunted by the knowledge that an individual suffocated or burned to death because they were allowed to ignore something so simple and fundamental whilst under their charge.