Robert Downey Jr. can do arrogant, confident, genius - and addicted.
I considered Jason Isaacs for the role originally, based on his ability to sneer and speak through clenched teeth while his blue eyes stab you through the heart. I think he made an awesome villain in the Harry Potter series, but COS is not Harry Potter.
Time is marching on and I no longer think Jason could play the role of Nez, who, while older than the woman he covets, is can't look that much older. I may consider him for another role in the COS series. I know whatever part he played, he would do it well.
Perhaps as time passes, Robert Downey Jr. won't be able to fill the role of Nez either. I know he can play self-absorbed playboy. If you saw his performance in Iron Man series you understand what I mean. The character Nez appears in all nine of the COS main series and they would have to be made in a remarkably short period of time so the actors would have to be really spot-on for the performance and endure well.
Just as all my good guys have at least one flaw, all my bad guys have one redeeming quality. For Nez, you'll have to wait for it because it is coming - even though he's not aware of it. Don't blink or you'll miss it.