How Much Finished Game a Trailer Editor Needs
How much of the game has to be done in order to make a trailer? I'd say surprisingly little, provided you have the means to create shots using other tools which don't require the game to be programmed, running in realtime with full lighting, visual effects, sound effects, music, voiceover, etc. By tools I mean things that are made for traditional animation, CG animation, 2.5D animation, and other ways to create narrative scenes or simulate gameplay via art and animation.
The real answer is of course: "It depends."
It depends on how much you need the trailer to say about the game and how skilled your trailer editor is at milking what little you have available. I think a professional trailer editor will have the skills to take almost any amount of finished game and say: "I'll make as good of a version of what you want using what you have" and adjust either your expectations after they've made and assessment, rescope the trailer's direction or advise you to wait until you're ready.
This color palette didn't really end up being in any moments from the finished version of Firewatch, but it certainly did its job for the teaser.
In general, you will need more game to be finished before you start making a trailer if your game:
Has low production value
Lacks a unique game design hook
Doesn't have strong art and audio design
For example, Firewatch made its initial teaser with very very little finished game, but they had a killer team with great art direction, great music, great actors and writing. Also, the game is not the sort which is built upon flashy gameplay mechanics and interconnecting systems, so a lot of stuff could be made custom for the trailer if necessary.
I wasn't involved in making the trailer for In the Valley of Gods, but I imagine it was a similar process where they had a great art team, a unique setting and cast of characters, they licensed a great music track for the trailer, and of course they had their pedigree from the success of Firewatch.
I don't know if any of this footage from this trailer for In the Valley of Gods represents actual player input, but the setting is crystal clear and there's a general sense of what you'd do in the game.
I've worked on game trailers during all steps of production, from when the developer is pitching to publishers but have barely programmed a single thing, all the way through to DLC updates and things after the game's release. This means I've worked with no game, very little game, a good chunk of game, and the finished game.
For an announce trailer it depends on the game's focus. For a narrative game you need enough to establish the premise of the world, and/or the characters' identity or predicament. For a gameplay driven game you need to at least establish the genre and show some of the gameplay hook in order to differentiate it from similar games. One complete game loop might be enough, but it really depends on the design.
For example, Little Kitty, Big City was able to steal hearts with assorted scenes of the cat doing cat things, even though they weren't part of a completely designed loop or level. But that's because the visual appeal for the game is SO high and of course people already love kitties and there aren't many games where you play a cat :3
For a gameplay trailer you'll want enough to show the game loop in several different scenarios so people can get a good sense of the variety and nuance of the core loop as it's explored and expanded in the game. A story trailer will need enough story written and maybe voice acted to give a bigger sense of the plot, what the characters do in the game, their obstacles, etc. As far as story, it should be at least enough done to make a voiceover specifically for the trailer even if enough hasn't been recorded for the full game.
Some games just don't have to show much in order to let your imagination run wild with the possibilities :3
Again, this all highly highly depends. For example, it will generally take less work for a smaller indie game to look "done" than for a big budget AAA game with a level of graphical fidelity only attained through tons and tons of time spent on motion capture, animation, lighting, modeling, visual effects, programming, etc. This is why AAA games will generally not show gameplay trailers until at least a year or more into development.
Conversely, a small indie game with little to no hook will need to do a lot more in order to get people interested. If you've spent years making a Japanese style RPG game with all the staples like: turn-based combat, villages, NPCs you talk to, shops, weapon upgrades, and an overworld, BUT nothing stands out about the world, the premise, the characters, the systems. Then it might not be ready to show until the differentiators are really fleshed out.
If you're not sure you have enough game to make a trailer, at least get an assessment from a good game trailer editor. We're used to trying to make a lot out of very little, and if we look and say "Uhhhhh, you don't have enough yet for the trailer you want to make..." then that might save you from coming out of the gate before you were ready.
