1.

For years cowboys and Indians were the staples of the screen.

used to

For years cowboys and Indians USED TO be the staples of the screen.

2.

Hollywood has been dependent on the western for its continued popularity and success.

owed

Hollywood has OWED its continued popularity and success to the western.

3.

Hollywood has been dependent on the western for its continued popularity and success.

due to

Hollywood's continued popularity and success has been DUE TO the western.

4.

Film-makers understandably lose much of their interest in these genres.

attract

Understandably these genres ATTRACT film-makers much less.

5.

This assumes that the crucial factors are those of technique.

ones

This assumes that the crucial factors are technical ONES.

6.

We can see only what they permit us to see.

let

We can see only what they LET us see.

7.

Each of these explanations is probably valid to some degree.

validity

Each of these explanations probably has some VALIDITY.

8.

The audience is an active participant.

part

The audience takes an active PART.