You can make your questions mandatory or optional by toggling the "Mandatory" slider.
Blue = Mandatory
Grey = Optional
When a field is mandatory it will display with a red asterisk next to it in Casting.
If the applicant doesn't populate a mandatory field, they will see the following message, and won't be able to progress through the application until they populate it.
See the Casting Help Guide for more detailed information on configuring your application.