Making your game trailer like a tutorial can feel dry and boring if not done correctly. One solution is to add a narrative layer for the information you're communicating to the audience; this is done to great effect in the trailer for Andy Schatz's game "Monaco: What's Yours is Mine" (trailer made by Kert Gartner).
The trailer is framed by a diegetic narrator who is assembling a team to conduct heists. When something is diegetic that means that it exists in the context of the story. A classic example for something that is non-diegetic is a musical soundtrack. In Uncharted: The Lost Legacy, Chloe can't hear the non-diegetic music playing while she's on her adventure. But in Sleeping Dogs, Wei Shen can hear the diegetic music playing from the radio of his car.
In terms of this trailer, if the narrator said "Monaco is a video game where you conduct heists!" that would be non-diegetic because that narrator is speaking as a voice that exists outside the story and game. But since the trailer uses a diegetic narrator, it has a means to describe exactly what you're doing in the game, and keep it in-universe.
After the introduction, the next section starts: "First, bring me a locksmith..." This narration not only tells us there's a locksmith character, but the word "first" lets us infer that there's more to come. Assuming the audience is engaged, the question of "What other character classes are there?" gives them reason to keep watching.
Something I didn't remember until rewatching this trailer is that the pretty motion graphics for all of these sections contain a description of the characters' abilities under their title. This is nice in that it provides more information, but I doubt there's enough time for the audience to properly digest it, because the voiceover is competing with the text for their attention. Any time there's onscreen text that isn't read by the narrator, you risk dividing attention, which means you risk losing attention. Best case scenario when you do this is the audience can process all this information at once, or maybe they'll go back and pause to read the graphic, but I think it's more likely that some information will be lost.
After the section for The Pickpocket and The Cleaner is the only out of universe graphic: "Play Solo or with 2-4 Friends Locally or Online!" You might wonder: "Why wasn't this made in-universe too?" This is because the diegetic narrator can only talk about what's in the story, so if they suddenly said: "Play with your friends" that would've broken the conceit. The game Monaco does not exist in the world of Monaco, so it cannot be referred to by the narrator who exists in the world. This is a necessary exception to make, because it's very important information that the audience needs to know about the game.
Finally, the narrator briefly mentions the other character classes (there's simply no time to dedicate an entire section to all of them), sums up his wishes and ambitions, and the trailer is over!
Hopefully the audience finishes watching this not only knowing roughly how the game is played, but with a feeling of the game's world and atmosphere. Games are often described like products by non-diegetic narrators (a good example are some of the Grand Theft Auto trailers), and sometimes this works just fine, but I tend to find that sort of approach rather dry, it really depends on the game.
Thus far, all the Supergiant games have some sort of narrator, which is of course perfect for making an in-universe trailer! This is in contrast to Monaco where the narrator exists in the trailer, but not the game.
The narrator for Pyre's trailer actually treads the line between diegetic and non-diegetic because the first thing he says is "From the Creators of Bastion and Transistor. Supergiant's previous games, I presume are not things that exist within the fiction of Pyre, but they have a good reputation, so it's important to include. Despite this first line, the rest of the narration tells the player what they'll be doing, but at the same time, it works perfectly well as something the in-game narrator might tell the player character.
Oh, and because this is Supergiant Games, the music, art, and motion graphics are just jaw-droppingly gorgeous, so this trailer has just about everything going for it. The only thing I would change is to eliminate some of the text in the dialogue boxes; the chosen dialogue is all quick and snappy, but for example: "Hedwyn beckons you to enter the blackwagon" next to the text "STEP INSIDE" is too much to read in the time given. But this is just sheer nit-picking.
If you have some sort of narrator in your game, not using them for your trailer is a missed opportunity; anything you can do to make the trailer feel like a companion piece to the game is worth pursuing!
Hopefully this gives you an idea for how a diegetic narrator can serve the double purpose of teaching the player what you do in the game, while also giving a sense of the game's world. This approach won't work with all games, so it's up to you and/or your trailer editor to figure out what is best.
In my opinion, narrative heavy games benefit from diegetic approaches. Whereas games that are more about the player's experience, and not the player character's story benefit from a non-diegetic approach. Or if your game has BOTH a strong narrative and a lot of expressive gameplay, it might be worth making multiple trailers.
In Part 3 I talk about trailers framed by non-diegetic narrators, that say up front: "Hey, this is a video game, this is what you do." Read it here!