I've ridden stoker, and its probably easier for a non-rider than for someone who rides.
The instinctive thought for the experienced rider is to move their body weight around to balance. That would work if the stoker and captain were completely in tune and had the same riding style.
However, cyclists have different styles and move the bike differently. For me, being on the back with a MTB rider on the front, it felt like the bike was falling over a lot. Thus I'm more of an upright touring style of balance.
An inexperienced rider as stoker might work better, if they lock their arms and keep their center of mass straight over the centerline of the bike.
As for an unexperienced rider, perhaps your first ride should be completely away from roads. Spend the first 10-20 minutes tootling through a park or somewhere completely isolated, to get to a first step of comfort.