Let me introduce you to Colin O'Callaghan - the antagonist in On Jacaranda Street. He's a violent Sydney underworld crime boss. Pete Mulligan is part of his gang of art and curio thieves, and once Pete comes under Jack's protective wing, Jack Collingwood becomes a target of O'Callaghan's anger.
O'Callaghan is convinced that rough-cut diamonds are hidden at Currawongs' Crossing, where Jack and Bea are staying with Dr. Miller and his spiritualist wife, Madame de Clermont. Why is O'Callaghan so convinced diamonds are somewhere in the house, and who convinced him of this? You'll have to read the novel to find out!
But what I can say is that Pete, Jack, Harold Chesterfield, and Julian Bartholomew all become involved in O'Callaghan's obsession. The crime boss will go to any length to find the gems, including murder, and Julian becomes implicated.
I always have a real-life person in mind when I write. It helps me imagine a character's physical characteristics and mannerisms. For the art dealer, Julian Bartholomew, it was a mix of Alexander Skårsgard and old-time Hollywood actor Richard Widmark. For Colin O'Callaghan, it was easy - Colin Farrell, who is one of my fav actors. He can play any role from street thug to detective to Alexander the Great, so I absolutely could see him as Colin O'Callaghan. Bonus: they share the same first name (unintentional, I might add).
Source: Wikipedia
Writing from O'Callaghan's perspective allowed me to get into his troubled mind, and I have to say I enjoyed writing from his point of view.