- What is the point of you being in this scene and what do you achieve, directly or otherwise?
- Would you be missed in the scene/story if you were written out and, if so, is the impact decisive enough to justify you're going into the story in the first place?
- Do other characters depend on you and, if so, why?
- If you're not the hero or villain, what is your purpose?
- What are you adding to the story?
- Do you learn from the mistakes you make in the story and therefore contribute to the tale that way?
- Are you the "warning to others" character or the one others inspire to be like? Equally, are you crucial to the hero's/villain's success (or otherwise!)?
- What are your main virtues and faults? (No cardboard cut out characters here, thank you!).
- Do you help drive the plot? (Crucial thing here is not to bring it to a halt!).
- Is the information you have/find out critical to the plot?
I think by the time you got even part way through this list, you'd have worked out whether your character was performing strongly enough.