The interview is you with a Captain and an HR person and is divided into a behavioral and then a technical portion. The behavioral protion is 5-6 TMAAT questions from your professional past to assess your decision-making process. The technical portion is walking through a hypothetical flight with you as the Captain. They'll provide your some prep paperwork to have more or less memorized prior to the interview for this portion. You'll be asked a few generic technical knowledge questions, be confronted with a small enroute abnormal procedure, and deal with a diverstion at the destination. It's nothing too difficult for someone who has flown 121 for a while.
https://www.aviationinterviews.com has many descriptions of what the interview is like from pilots who have gone through it, including whether or not they were hired. It's around $40 for a month of access and I found it very helpful during my preparation (don't forget to unsubscribe when you're done with it to avoid getting charged again).
I also used Cage Consulting. Their interview prep package is $450 for an ALPA member and was very helpful in identifying weak spots. It's pricey, but in my opinion worth it (how much are you willing to put into getting this job?).
This and other threads have a lot of information about the interview. I'd go back and read through them.
I'm soon-to-be former ULCC, so feel free to PM if you'd like.