Ideas can happen at any time, sometimes when you least expect it. It’s always a great idea to keep some paper and a pen nearby just in case you do get an idea for an awesome creation!
I really enjoy one page game design documents that can tell a person (or maybe, mainly you) what your idea is and the basic mechanics for your idea. This way you don’t waste time, which could mean a lost idea, and you get your idea on paper in its most basic, purest form.
Here is an example of a game idea I just got from laying down for nap. I’m going to give myself a bit of time to implement the idea and then make it a playable game, which I’ll post here as well.
As you can see it is pretty raw, but it gives me an idea of how the game will look, the mechanics that may be in the game, and some simple ideas I had in the moment. This gives me a clear direction on what I need to get started and doesn’t make the task seem daunting.
Once I get the basic implementation made I can then add more features to it, get people to test play it and give their thoughts, etc. Be warned, sometimes after you make a demo of a game you may get less than positive feedback, or maybe the spark of the idea burned out. It happens, so instead of wasting time trying to make a game that you do not like anymore look perfect you can move onto the next one and keep the fresh ideas rolling.
But, if you have the spark to make it something you can show in an interview, do it, and be proud of it!