1) Do what it says here for any characters who really matter :
http://emmadarwin.typepad.com/thisitchofwriting/2013/03/characterisation-in-action.html
2) Imagine an encounter between two of them - either a situation from the novel, or something which happens off-stage, or something completely different.
NB: In some ways, something not-in-the-novel is the most fruitful, becase you're more free: you're not having to make it fit the engineering of the plot and story, and can let your imagination run with its instinctive sense of who these people are, which could go anywhere...
a) Write it in first person, in the voice and the PoV of one of them.
b) Re-write it in first person, in the voice and PoV of the other.
c) Then re-write it in third person, in the voice of the narrative but allowing the PoV character's voice to colour that, switching PoV once wherever in the scene it will have the most effect.
Edited by EmmaD at 16:06:00 on 30 September 2013