Pay only if the see your ad; must see the ad for 30 seconds or the length of the ad to get charged for the ad. So if they see your ad for 15 seconds and the ad is actually 60 seconds long, you will not pay.
Call to Action Overlay (not part of creative) – Set this in YouTube account
For example, Cars. In-Market are those who are actually trying to buy cars. Affinity are those who are just car enthusiastic about cars
Better to create several creatives and show different creatives and cap each creative.