HC, its not about how you 'should' behave... as you say there is no 'correct' way ...
its about behaving 'realistically'
people trained in reading micro expressions, body language, language experts etc are well aware that shock makes people act differently, they can account for that, but there are many signs that people faking simply cannot control or simply over look.
they dont look at the expected actions - their job to look past the obvious signs - the ones amateur actors would think about ...
its similar to the way humans cannot alter the results of a lie detector test - because they are testing things that we cannot control.
your state of shock in your circumstance i am sure although you may have thought you were acting odd... all the little giveaways would not have been there, so you would have been believable.
these two look as though they are just thought 'look miserable and wipe our eyes a lot' - because they think thats enough.
but in these cirumstances police look at the stuff they arent doing.
speech also plays a part here, phrases, tenses, choice of words comes out offten subconsciously... stuff like using the past tense when referring to the missing person .. "the used to go", "they liked this" etc - it gives away the fact that you know they are already dead - if you genuinely were unsure you would use present tense.