The first step is to establish the reality of the threat. A good way to do this is by showing the impact of flashover and the speed with which fires can reach flashover. Resources like the RFSI video use live demonstrations to depict what happens to people id dwelling fires. Flashover is the critical point because of its impact on victims and fire suppression resources. Fire loss data show that post-flashover fires increase the risk to people outside the room of origin by a factor of eight.
Stopping a fire before flashover, then, dramatically reduces the threat to occupants in other parts of the dwelling. Victims in the room of origin are affected about half way to flashover because this is when the temperature, smoke and carbon monoxide make the room untenable to life. Residential sprinklers stop the fire before it makes the room untenable to life, so they dramatically increase the chances of survival in the room of origin as well.
When describing how residential sprinklers are so effective at stopping flashover, reinforce the fact that before residential sprinklers, flashover and the ensuing life and property loss were mostly unavoidable, an acceptable loss if you will. With residential sprinklers, we don't have to accept that anymore.