The others have given good answers, I'll just elaborate a little on в хмеле / во хмелю
Secondary prepositional forms in -у, -ю (and stressed -ы, -и for some declension I nouns), while technically being prepositional, in fact mean locative.
You should use these forms when в ("in") or на ("on") means "being there", literally.
Let's consider some examples:
Предки Грегори Пека видели в лесе препятствие
Предки Грегори Пека видели в лесу препятствие
Both phrases translate verbatim as "saw in forest (prep.) hindrance (acc.)".
The first phrase means that Gregory Peck's ancestors, the first white settlers in America, treated the forest like a hindrance and burned it to create a patch of arable land.
The second one (constructed by me) means that they were, probably, walking down the forest and saw a hindrance or an obstacle of some kind, maybe a stump or a fallen tree.
We can reword the second phrase as "Gregory Peck's ancestors saw a hindrance while being in the forest", but not the first one.
Hence, we use prepositional proper in the first (лесе, not "being"-like), and locative (лесу, "being"-like) in the second one.
В «Мосте Ватерлоо» с Вивьен Ли (Успенская) сыграла русскую преподавательницу балетной школы.
This translates as
(Ouspenskaya) has played a Russian ballet teacher in "Waterloo Bridge" starring Vivien Leigh.
Again, you add "while being" here: the actress played in the film "Waterloo Bridge", but not "while being" on the actual Waterloo bridge. Hence, prepositional proper (мосте).