Okay! This is about matching clips to the text. It might sound like a simple idea, but most people don't do it and by not matching them the trailer loses some of its effect.
If the text you're using for the trailer is: 'Meet Emily', you could throw any random clip of the actress who plays her and be done.
But that looks bad.
It's important to understand genre and/or themes of the story.
If it's a sadder depressing story, show clips of Emily looking sad or depressed. If it's a happy story, show clips of Emily smiling and laughing.
I know it sounds a little basic, but it's important to have.
Another example: If the text is, "Cali's been running her whole life." You wouldn't show clips of Cali smiling would you?
It would just look... well weird and out of place!
But you could show clips of a girl running? Or a clip of Cali crying or looking distressed?
Also, it's not only about finding a clip. It's knowing how long to use it for. There is not correct amount of time to use the clips for.
It's all based on your judgment. You don't want to have the same clip playing for twenty seconds. But then again, if it works it works. I know that made no sense. But as you go along you'll start to be able to notice when your clips to long.
I know this was a very short lesson! And it's kind of basic knowledge but it doesn't hurt right?
TIP: I always follow this rule, which I think is a good one to follow. Always show at least three different clips per text.