Clarity in Cases

Hi Everyone,

I am setting up cases in my environment and I want it to move to 2nd stage when the user fills in all the information within the 1st stage. 

I believe that it can be done through business processes. But when I am adding the Sub-process within the case, it asks whether to run at the start of the case. Does it mean that it will only run that process one time? Because if it runs at the start that means our users will not have inputted their data and after when they have added the data then it will not run the process and will not move forward?

Also, let me know how can I make a formula that checks if the fields are filled or not filled?