Choose the module e.g. Service Calls Select the Field and set if it is always required or required for a certain status.
This is the basic way to achieve this.
For more customisation you can use UI rules with a stop notification to stop the form from being saved if a particular field has not been filled in. This gives you the option to build conditions from the UI rule to achieve this.
tools --> System --> Security --> Prevention --> Required Fields
now, choose the required item that you would like to have changes, double click to open, choose the Field that you would like to make as mandatory and tick all possible status values where this field needs to be made mandatory, save it and then there you ahve it.