Writer's Fallout Shelter

Experience and empathy. You might be able to write credibly about a party girl drug addict in Japan if you could empathize with a drug addict, ever attended a wild party or two, and knew something (or did much research) on Japan and its "party scene" and drug use.

An uptight moralist who'd never experienced any kind of addiction, never attended a party, and looked down on anyone who did wouldn't write this book credibly. It would come across as shallow and judgemental.

To me, that's what it means to "write what you know." Empathy is key - not pity or judgement. The rest CAN be done through sufficient research. But to have real empathy, you usually need some kind of similar experience - not the SAME experience, mind you. I've never done illegal drugs, but I'm hooked on smoking and have had alcoholic friends. I've read books and seen movies that dealt with addictions to harder drugs. I've been to parties and watched women throw themselves at men for all the wrong reasons. I think it's Japanese culture I'd have to do the most research on; that's the part I can't "fake well" in your scenario.