Yes! It (where) probably would work in English, but in french we need to express "of whom" The film "of whom / of which" the actor .....
This is why "dont" is the most appropriate.
This is how I see it -- hope it helps.
où is a spatial or temporal preposition (where, when). As Jim says, in the example you inquire about, you are looking for a possessive construction (of which).
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