OK, so this is a bit sneaky, but Google recommend “Audience Expansion” on your Display ads, even if you use a remarketing list. This might be OK if you are going for a maximum reach campaign, but not for a Remarketing campaign. Why is that?
Remarketing campaigns show your ads to the same people who have interacted with you before, but an expanded audience is people who have never interacted with you before. If you “expand” the audience of your remarketing campaigns they are no longer remarketing! By doing so you have defeated the whole purpose of the campaign. If you accidentally put the expanded audience setting on your remarketing campaigns, don’t be surprised if your remarketing audience gets less than 1% of all your impressions and clicks on the campaign. You’ve been warned!