How much you need to brief your pillion will depend on their overall experience and, specifically, how much they’ve ridden with you.
It is always worth reminding passengers of the essentials: how to mount and dismount, only get on or off when instructed to do so, no feet down, no wiggling about, etc. For novices or less experienced pillions, you’ll need to explain the sensations they are likely to experience - that bikes bank when cornering and not to worry about it - as part of a full briefing.
Encourage your passenger to:
Develop basic signals:
Passengers are even more likely than you to be injured in a crash, so they should wear protective gear at least equal to yours. The rule is simple: If they don’t have proper gear, they don't ride.
Check over all your passenger's fastenings to make sure they are done up properly: helmet strap, jacket and pants, gloves and boots.