I am not sure what you mean by being physically in the movies.
- Miikka ja Hilla ovat elokuvissa.
Miikka and Hilla are at the cinema, they are watching a film.
- Miikka ja Hilla ovat elokuvassa = …esiintyvät/näyttelevät elokuvassa.
Miikka ja Hilla perform/act in a/the movie.
- Miikka ja Hilla ovat elokuvateatterissa.
Miikka and Hilla are in the cinema, they may or may not watch a film.
- Miikka ja Hilla ovat elokuvateatterirakennuksessa.
Miikka and Hilla are in the cinema building, they neither watch a/the film nor perform in it.
Elokuvateatterirakennus is a compound word of three parts:
- elokuva : cinema, movie
- teatteri : theatre
- rakennus : building
You put it into the inessive case to show "being in/inside". While most nouns end in a vowel in Finnish (in their sg. nom. form), there are some others. I think the letter "s" is the second most common. If you add the case ending directly to such a word, you get rakennusssa which is unpronouncable. The vowel "e" is the general "glue" vowel used in attaching case endings, so theoretically you could have rakennusessa. However, I can see a problem with that, when you pronounce it the syllables will be ra-ken-nu-ses-sa, i.e. the first "s" will no longer be in the same syllable as in the nominative form. I am not sure, but that sounds really strange for Finnish. So the last syllable of the nominative form must be somehow changed to have the inessive ending.
To complicate matters there are a couple of categories of nouns ending in "s". For the nominal type 39 the change is from "s" to "kse" where "k" remains in the preceeding syllable, i.e. the one with the original "s" and "se" begins a new syllable.
Some nominal type 39 words (here with the genitive inflection):
- vastaus → vastauksen : answer
- kynnys → kynnyksen : threshold
- kyljys → kyljyksen : cutlet
- kaulus → kauluksen : collar
- tennis → tenniksen : tennis
This basically translates as "Miika and Hilla are in the movie" good for them, personally il be elokuvateatterissa!
Elokuvissa is a modern application of the old structure of käydä marjassa/kalassa, which don't mean that you visit the inside of a berry or a fish, but you have gone somewhere to pick berries, to fish. Thus käydä elokuvissa means, that you go somewhere to see movies.
Olla elokuvissa is an idiomatic way of saying that someone is at a movie theater (or a similar place, not at home) watching a movie/film. It makes no difference how many movies are watched.
Olla elokuvassa means that someone is in the film as a performer.
Katsella elokuvaa is just generally watching a movie, at home or a movie theater or wherever.
There are three uses of that -lla/llä ending. The first one is literal, to be "on something", e.g. Leipä on pöydällä, The bread is on the table. The second one is possession, e.g. minulla on voileipiä, I have (slices of) butterbread. The third is more vague "at, in", olemme usein pihalla kesäisin, we tend to be in the yard during summers/at summer.
Given those three uses, what would elokuvilla mean? Surely the literal meaning is out of the question. The second, possession is possible for instance if you talk about some quality some movies have. Lännenelokuvilla on vanhanaikaisen lajin maine, The western movies have a reputation of being old-fashioned genre.
However here in the exercise we have two persons as subject, so the third interpretation kicks in, it must refer to a place. The problem is that elokuvilla does not refer to a place, elokuvateatteri would do that, so
- Miikka ja Hilla ovat elokuvateatterilla : Mikka and Hilla are at, near the cinema (bulding).
But this has a different meaning that the original one.
My answer "Miikka and Hilla are in the film" as well as the suggested correct answer "Miikka and Hilla are in the films." should NOT be accepted in my opinion. Both mean that these people are playing some roles in the films which is not what the Finnish sentence means (according to this discussion). What it seems to mean is similar to "olla marjassa" from the previous lessons and this construction simply doesn't exist in English so the translation should be something like "Miikka and Hilla are watching films". I just don't understand where the -issa ending came from.