Genre, Setting und Plot
The choices the player makes lead to branches that will result in different story paths. Therefore you will have to conceive and plan different plot threads and maybe also different endings, forming a kind of tree diagram. If you do not plan the setup of your story well, it can lead to your story feeling too one-dimensional or overwhelming your player with its complexity.
Also decide early on whether the player’s decisions will influence their understanding or completion of the story. What happens if the player does not receive some vital information or clues? Can the player still make sense of the plot? Can they still complete the story? And can they still complete it in a way that allows them to reach a satisfying ending?
So it is not just about planning the important story events beforehand, also make notes on where your player can find the triggers and solutions for these events. And make sure that they will actually receive all necessary information and items. There are several ways to achieve this. If a player did not receive or missed a decisive clue, you can have a knowledgeable companion or other character give it to them at a later point in the story. This will enable the player to keep playing, but at the same time will show they their error.
An interactive crime story might not work so well if the player, due to their decisions, never meets the murderer or finds the evidence that is needed to convict the culprit. And a closed door in a dungeon will generate frustration if the player cannot get free because they missed the key earlier in the game or failed to pick it up.
Therefore make sure that your story will not lead to any dead ends. This does not mean that the player should always be able to reach their desired goal. But the different endings have to be designed in such a way that they are each self-consistent.
For the crime story this could mean that a companion points out to the player the importance of a specific clue or piece of evidence necessary to convict the murderer. In the dungeon scenario, a companion could intervene and pick the lock from the outside. Or you just make sure that the player cannot reach the dungeon until they have found the necessary key.
However, such assistance for the player should always come with a price so as not to reward passiveness. There are no bounds for you in how you wish to accomplish that. Emotional consequences are particularly effective. The reactions of the other characters in the world could show them that them did not necessarily cover themselves in glory. Or you could deny them some game elements that are not important for the main story but would enhance the gaming experience (e.g. lore, side quests, Easter eggs, etc.).
If you want to offer your players an elaborate system of game mechanics – for example a combat system or an individual character creation – design it with great care and reduce it as much as possible. Depth is good, complexity is not.
Voice is a speech medium, and even though some output devices have displays that enable the sharing of visual content, words heard are fleeting. Therefore, in your first stories, focus mainly on the plot and from there begin to approach more complex game mechanics. You will gain just as much from this as your players will.
One-Dimensional Main Plot Thread
The player has no possibility to deviate from the main thread of the plot. Their choices do not influence the plot, only the way in which they experience it (or maybe not even that if you did not pay any attention at all to this guide).
The player takes on the role of a princess who marries the prince at the end of the story. This ending is inevitable. However, the appeal for the player lies in how they reach this goal, be it through flirting, playing the girlie girl, or having empathetic conversations with him.
Interactive stories with a linear main plot thread do not offer much freedom. They can still be successful by compensating for this limitation in other ways and allowing the player to act in different ways. This can be achieved through emotion. For example, each Player Choice could offer the player three different ways in which to reply, like “witty, cool, or romantic”. This enables the player to identify with the personality of the character they are playing.
Branching Main Plot Thread
The main plot thread branches several times and is merged again in different ways. The core events of the story are being dictated by the main plot thread. However, how the player reaches these core events depends upon their decisions. Thus the player can experience the main events of the story following very different paths, giving them an incentive to play the story again.
The player has to reach a certain city. To get there, they can either use a sailing ship, take the pass through the mountains, or ride on the back of a dragon. Depending on their choice, the player will experience different adventures on their journey, find different items, meet different people, and so on. During the voyage on the river, the ship is being attacked by pirates, in the mountains they have to take cover from an avalanche, and while riding the dragon, they spot a cave entrance beyond which treasure awaits.
Also, the events of these scenes do not have to be of equal value. A balance can be achieved over the course of the whole story, or maybe you deliberately want to treat these events in very different ways.