Note I did not take the Sheppard Interview Prep course so don't know what that covers.
The questions on the written seemed to be mainly (maybe all) out of the ATP written. That said, I took my ATP written a while back and under the old rules, no idea what the new written is like or if any different. There were a number of questions specific to Part 121 ops where I honestly didn't have a clue (I have always been a part 91 guy). Gave it my best shot.
Then was asked to brief an approach (jepp plate). I told him I wasn't familiar with Jepps and he was fine with that...just asked me to brief it as best I could. Asked me some questions about the approach (answers will be on the plate but may be somewhere odd). Also asked to brief a SID and a START. Again, no surprises. If you are current on IFR and know plates, none of it was a big deal. The Jepps are a bit different than NOS plates but not so much you can't figure it out.
Also asked to read and interpret a TAF and METAR report. Before the interview, go over all the METAR codes. He ended up asking me one that I didn't know. Told him I couldn't remember what it was but that I would look it up and where I would look up the code.
No questions where I had to do any fancy formulas (descent planning, time to a station or any of that stuff).
No systems questions in my case. I honestly think the point of the technical part of the interview was to determine if you were a competent IFR pilot who knows the rules and is familiar with the system and the tools we use in it (SIDS, STARS, charts, etc.).
For what it is worth, my impression is that, if you get an interview, the job is yours to lose. The company has already invested time and money to interview you. They would not do that unless they expected to hire you. They are really looking to see if what you reported on your application is BS and if you have the basic knowledge they require and if your attitude and demeanor is someone they want to hire.
I would also have some questions prepared to ask during the interview. You will be asked if you have any questions. Have some intelligent questions to ask..it is your chance to ask what matters to you. Especially if you are interviewing at multiple airlines and will have to decide which offer to accept. I asked about domiciles, impact of the new base at RDU.
Good luck!