70 years a quite a long period to cover. My concern would be that you end up doing a blow by blow chronological description of what happened in, say, each decade. WHich is not the best kind of dissertation.
However, it's a start - so...
- what 70 year period are we talking about?
- can we book end it with a classic action movie at either end, each being representative of the movies being made at that time
- how will you break the 70 years down? rather than go withd decades, - can you identify aset of films that represent a new developments or paradigm shifts with protagonists within the genre across that seventy years- so one at either end and maybe another 2 or 3 in between?
Can you see where I'm going with this?