Is the difference between these two sentences, just simply that the film had already begun when the person arrived in the second sentence, whilst in the first sentence the film had come unto the screen, just as the person was sitting down, or ready to watch?
When I arrived the film had started.
When I arrived the film started.
The Past Perfect expresses the idea that something occurred before another action in the past. It can also show that something happened before a specific time in the past. Therefore, the first sentence you wrote is incorrect!
I think you can use both of them. It depends on what you wanna mean. If you go to the cinema and the film starts in the moment you sit down, you use "started"; if you go to the cinema and the film has already started, you have got to use "had started" because it's an action happened before you sat down.