It’s pretty well impossible to judge how an audience is going to react, and where they might or might not applaud, at least in Mozart; perhaps it’s a bit more predictable in romantic works after a stonking great great high note, but Mozart is a bit more subtle. Even if we assume that there will be applause, different audiences will be in different moods, and will be more or less enthusiastic. One thing I’ve noticed, however, is that there are occasions where the performers and the audience end up in a situation where they are each waiting for the other to finish before they move on, and the big one is kissing.
If you sing a duet that ends with a giant smooch, it’s inevitable that the audience will keep applauding so long as the singers keep rubbing their cheeks together (while apparently attempting an impersonation of a bobble-head); and of course, the singers have inevitably been directed to keep smooching until the applause dies down….
You can see the problem!