When you complete actions in a task for example in a fashion show, the you only have a set amount of energy to pose and face the camera, each action takes a certain amount of energy, this energy does refill over time, but at a rate of one block every five minutes only. You can swap your K starts for energy if you want.
The idea on tasks like a fashion show or a date is to achieve five stars within the set time period, for example one hour after the event starts. Therefore you have to utilise your energy to attain those five stars within the time period.