The string "Some of them write the scripts for, act in, and even produce their own motion pictures" is an independent clause. You can't just drop it into the middle of another independent clause "Contemporary film directors are thereby assuming ever more control of their art." You have to either coordinate the two clauses, by juxtaposition or conjunction ...
JUXTAPOSITION: Some contemporary film directors write the scripts for, act in, and even produce their own motion pictures. They are thereby assuming ever more control of their art.
CONJUNCTION: Some contemporary film directors write the scripts for, act in, and even produce their own motion pictures, and are thereby assuming ever more control of their art.
... or subordinate one to the other with a subordinating construction such as a relative clause:
Contemporary film directors, some of whom write the scripts for, act in, and even produce their own motion pictures, are thereby assuming ever more control of their art.