Hello,

The final marks are the combination of interview and mains marks. So there is nothing like falling in UPSC interviews. UPSC has a certain cutoff for this total result.

In case if you fail(which will be a very disheartening thing) you'll have to go back and gather enough courage to start from stage 1 which is the preliminary stage. Try to score well in mains so that even if your interview doesn't go as expected your mains marks will balance out those.

In the post about you are talking about there are other exams for those that don't count in the UPSC examination.

Hope this will help, All the best!!