Vorhi: Watch even clips with Jackie Chan in them, or read the webcomic I pulled his playby from.
Mr. Ash: Angry, loud metal. Punk rock. Music that says "break stuff."
Olidiv: Something "fighty." Dynasty warriors, Black Mages, and other big, sweepy, dramatic stuff gets me into his head.
In general: Just rehearse conversations with other characters (even ones I haven't RP'ed with) in the thread. Read other threads. Sometimes getting into the weirdness of the universe helps me understand the character better. Like, for example, realizing that Vorhi was raised on his species second homeworld, and learned to fight on a completely different planet, and now works on another one. It helped understand his overall lack of permanence in things.
Final exercise for pushing a character's mind closer to mine: Imagine said character has been forced by a god or demon to sing on stage. What song would they pick, and why?