Upvote:2
You cannot easily extend a type C Schengen visa (you can find previous questions about that for details). You have at least two option:
Leave right now (or at least well before the end of your 90 days) and return when you need to be here. The clock stops, as it were, and if you have a multiple entry visa and it's still valid, you can use those 90 days any way you like, not necessarily at once starting from your first entry.
One issue however is that it's not clear that shooting a movie would be allowed for you. Working is in fact allowed under a Schengen visa but you also need to secure a separate authorisation for that.
Apply for a German (long-stay) work visa. It's complicated and I am not sure it would be possible to get one in your situation but if you do get one, you kill two birds with one stone: the 90 days are not relevant anymore and you don't need to have any concerns about shooting a movie. You would have to leave Germany first and apply for it from your place of usual residence.
(In some very specific circumstances, it is also possible to apply for a residence permit directly from within Germany β instead of a long-stay visa β but I don't think this would be possible in this case.)