Let's imagine we want to create a robot for a game and are given all the freedom we want (which will probably only be the case in personal projects, but it's a good example of how things could work). So we write down short, let's say three, sentences to better define our character.
1. Let's imagine there is not just one, but two robots guarding a city for over a thousand years.
2. Both of them were given orders at the time of creation, but are not to be controlled any further.
3. They haven't moved since their creation, so they probably won't in the future, it's just a tale, right?
Let's see how our imagined robot takes form with each sentence.
"create a robot for a game"
At this stage we don't have much of a clue what the robot could look like.
1. Let's imagine there is not just one, but two robots guarding a city for over a thousand years.
Now we could imagine two robots, maybe like brothers in arms, big enough to guard a whole city. They must be strong, but also protective, so that bright, positive colors in combination with white come to mind. Maybe they have features of a knight being loyal beyond death. They could even feature building parts of the city itself as being a part of it. As for the posture, something very calm and resting could fit. Maybe they are sitting opposite to each other, awaiting their call.
2. Both of them were given orders at the time of creation, but are not to be controlled any further.
This may result in a neutral, machine-like expression on their face, following orders, without emotion and questioning, for ages.
3. They haven't moved since their creation, so they probably won't in the future, it's just a tale, right?
This even tells something about their future movement, maybe they move like a child, who has to learn walking and is struggling to keep the balance. It's crucial to bear in mind how the character would move while creating or modelling. Don't waste your time by modeling a character that can't move properly in the end.
The second part of the sentence also implies how others feel or think about the character, giving some hints about the background story and even creating some air of mystery with the uncertainty of their power.
When coming up with concepts for mediums like film or games, which have a high demand for emotions, it's often helpful to also express them in writing style.
This was just the beginning. Depending on the rest of the storyline, especially the period these characters live in or even the target audience, many different robots could arise from this short concept.
However, only three sentences made up a much clearer image of what we would like to achieve. With such goals in mind, you can always step back and check if your digital sculpture serves the same purpose as the concept. This will greatly improve your final work, because every part of it will serve a purpose, defined in the beginning. Searching such a purpose in other digital characters is also a great exercise.
It's also important to notice that a concept not only tells you what to do, but also what not to. Nonetheless, it's just a starting point, and it will evolve during the whole process.