I seems that there are catagories of questions they will ask you. For example: You will get asked a Divert/Fuel story, Procedure/Regulation story, a Leadership story, Emergency/Scariest flight and definetly Why SWA should hire you or What can you bring to SWA.
Divert/Fuel - Normal divert story, Min fuel, Low on fuel.
Procedure/Reguation - observing someone breaking rules, you breaking rules, did not agree with a procedure, changing a procedure, rug dance with a Chief Pilot.
Leadership - what leadership roles have you had, when did you fail as a leader, when have you had to take another pilot under your wing, when have you had to use diplomacy at work, when have you had to counsel a coworker.
Emergency/Scariest flight - speaks for itself. What happened, how did you deal with it, outcome, what you learned.
Also, customer service story. When have you gone above and beyond for your company.
The cool part is that if they ask you a question you don't have an answer to, they just go on to another question.
Read Nuts it will give you a feeling of SWA culture.
I went to southwest.com and looked at all their history and background also.
Consider using Judy Tarver with Higher Power for your prep. It is well worth it.
Good luck.