I talk a lot about the acts of a trailer in broad strokes, but I want to dig deeper on each act, starting with the Cold Open.
"Cold open" is a term that originates from film and television; it's meant to hook the audience so they'll keep watching. They were especially important for television since it's so easy to change the channel. According to Wikipedia, cold opens became common practice for television shows in the 1960s with shows like Star Trek. Just think about all those TV shows where the music flourished as the scene faded to black, then went to the title sequence. Nowadays fading to black is out of style, so TV shows like Breaking Bad have hard cuts to black.
As I mentioned in this post about the trailer's line of rising action, cold opens are more important than ever because of how quickly people can scroll past a video on a social media feed or click away to another video on YouTube. In YouTube's analytics you can even see a graph of how well your video retained the attention of the audience.
If you study cold opens for film and television, you'll learn a lot about trailer editing, because by necessity cold opens are:
Require little to no context
For a cold open I look for a scene either: heavy on action, funny without story context or visually very striking. For example: the cold open for The Frighteners, the Matrix trailer, and the Bad Boys II trailer all start with chases; I also started my Uncharted 4 fan trailer with a chase. A chase is a story unto itself because you wonder whether the person being chased will get caught.
Another example of a cold open that tells a story very quickly is the one I made for the Xbox One version of Firewatch. The sight of fireworks held next to a smoldering fire pit has suspense to it, because you want to know whether or not the fireworks will get lit. A way to evaluate whether a scene is a good candidate for a cold open is to ask:
Will the audience want to know how this scene resolves, and will it resolve quickly?
The other way to make a good cold open is to simply entertain with humor. I talked about this a lot in my post about stand up comedy and trailers, but a good opening joke will earn you a lot of good will. This was my approach for the E3 trailer for Firewatch. That scene made a good cold open because it showed a little bit about the characters, and ended with a little humorous moment. This first scene from the red-band trailer for Pineapple Express isn't really a cold open, but it always makes me laugh; I want to see more after that scene.
But what about trailers for games that have little to no story to them? In those cases, very good, exciting and CLEAR gameplay can be a good cold open. This was my thinking for the recently released gameplay trailer for Spelunky 2. I've read enough YouTube comments for game trailers that I know a certain vocal part of the audience just wants to see gameplay, and as soon as possible. Opening with gameplay in this fashion is my way of hopefully placating them, and fostering some good will so they'll stick around through some story before I get back to gameplay.
So how do you decide what makes a good gameplay trailer cold open?
A good way to think about it is:
What gameplay would you show as an animated GIF?
Animated GIFs have no benefit of sound or narration, and typically have no text when used to show gameplay. Therefore the visuals have to be crystal clear and comprehensible. If you could easily take your cold open and make it into an animated GIF, then you're probably in good shape.
Cold opens aren't a one-size-fits-all solution to trailers, but a well made one makes for a very fun trailer. If you're planning on starting your trailer without one, your visuals and/or music have to be top notch, or you need a captive audience, or a dedicated community, or some other reason for them to not look away. Like anything in editing, whether or not to apply this technique is part of the craft, and should not be treated like a guideline.