A great trailer can generate a lot of interest in your game. On the flip side, a poor trailer can turn people off before they have a chance to know your game. On Steam, the trailer is one of the main ways people are viewing your game and it needs to grab them. One of the ways to do this is to tell a story. The point of a trailer is not necessarily to show all the gameplay modes or features of your game, you want to get the player interested enough to learn more. I want to look at three trailers where I think this was done well.
This War of Mine
This is one of my favourite trailers, because it really captures what sets This War of Mine apart from other games. The tag line 'In war, not everyone is a solider' is really what the game is about. Trying to survive as a civilian with a war going on around you. Most games with soldiers place the player front and center in the action, and the trailer starts this path with the slow mo soldiers fighting. This is discordant with the melancholic music playing which makes you curious of what is going to happen. When it transitions to the family and the tag line the sound comes in and you get a feeling for how scary it would be in that situation.
This war of mine trailer doesn't show any of the game play, but it perfectly captures the theme and feeling of the game which makes you want to learn more about it. The actual game builds up much more slowly which I think is harder to convey in a trailer. Once you have played the game, this trailer still resonates hard because you have the full experience of what it is going to be like for that family. The main take away with this one is that you can get across the theme of your game in a short story, without having to show any gameplay.
Astroneer
The reveal trailer for Astroneer shows the game in an early state, three years before their official launch. What I like here is the arc that it takes showing off the different parts of the game. The initial 12 seconds of running combined with changing the scene in time to the light piano really draws you in and makes you curious to see more. The next part shows the exploration and digging with a whimsical music leading into the base building. So far a delightful game. Then we hit the downward part of our heroes journey as the music gets darker and we have the awesome shot of getting blown in the sandstorm. Finally we come out of that with meeting a friend (multiplayer!) and seeing riding on vehicles and out to space.
This gives a great overview of the various types of things you will be doing, while matching the theme of exploring and building where the enemy is the environment. For the launch trailer they follow a similar story arc, obviously a lot more polished with their game ready. I still find that opening 12 seconds of the reval trailer one of my favourite trailers I have seen.
Titanfall 2
For the final trailer lets look at a big budget AAA game. The Titanfall 2 Single Player Cinematic trailer nails the combination of theme, setting and showing gameplay elements. Set from the point of view on a lowly grunt on the battlefield, overwhelmed in an explosion you get to see a 'Pilot' save the day. It builds up showing the power fantasy of how bad ass the player will get to be as a pilot, cumulating in jumping in a titan. The last third they get in the story of the setting, with the melancholic rather than exciting music. This is in contrast to the more traditional high paced music of the gameplay trailer which I find much less engaging.
Trailers
A trailer has only a couple of minutes to get across the idea of the game. These three games have approached it in different ways, but all of them have shown the setting and the theme. It is not necessary to show the player everything they will get to do, often a 'gameplay' specific trailer can provide that information. The goal of these three is to hook the player in and make them want to learn more. Think about your favourite trailers and look at why you are drawn to them. Hopefully this will allow you to make a great trailer for your creation.