If they invite you to interview, they already believe you are a knowledgable and qualified individual. Use this interview to let your personality shine through. They have already hired your resume, now let them hire the person. If you are humble, honest, personable and professional there is no reason for them to turn you down. It is not a difficult interview by any means - short written test, few systems and jeppesen questions, sim eval and an HR portion. The gouges online are almost 100% accurate. The people you will meet are very nice so don't be afraid to crack a joke (if appropriate), say you don't know or ask for a second try if you think you made a mistake in the sim. Relax and don't let your nerves get the best of you. Worst case, learn from what you did wrong in this interview as a learning experience to do better in your next one.
It also might help to stay away from this website. Some of the attitudes on here are the exact opposite of what you should be taking with you into an interview. There are a lot of weathered individuals on here that get lost in the politics and bickering and have nothing meaningful to contribute. You will enjoy flying flying for a 121 carrier, let the interviewers know how enthusiastic and excited you are about the oppurtunity. There are enough disgruntled employees in the airline industry, I don't think any company is looking to hire more.
Good luck and let us know how it goes!