First of all, apologies if any or all of the below is completely redundant. I don't know your experiences conducting interviews like this so I am assuming no to little familiarity with the process. Feel free to ignore this post if everything I am saying is redundant.

You might get some good answers posting in /askanthropology also. Conducting these sorts of interviews has often been the domain of anthropologists collecting "oral histories" (as opposed to "written histories").

Generally though, these sorts of interviews are less structured and more influenced by what your informant (your grandfather in this case) are willing to talk about and what they think would be important. You can ask more directed questions, but what you think (or what /askhistorians thinks) is important may not be the same as what your grandfather thinks is important about his experiences. Sometimes, the most exciting information doesn't come from your line of inquiry but what your informant might think is most valuable.

I'm sure someone else with actual expertise in WW2 can help you with some more specific historical questions, but I just wanted to chime in with the more anthropological viewpoint. Go in with some questions of interest to help guide the discussion but be open for the interview to go places you don't anticipate. This is especially true when interviewing those who suffer from trauma - they may be reluctant to speak about their experiences so you may have to go slow at first by allowing them to only speak about what they are comfortable with.

All that said, being familiar with the outline of history in Indonesia in WW2 is really crucial to understand the context of your grandfather's experiences.

Here are a few resources that might be helpful:

Principles and Best Practices from the Oral History Association

Handbook on Oral History from Baylor University Institute of Oral History

Do you speak Dutch? There's a great deal of literature in Dutch from camp survivors who later became novelists, and as a writer you might find that as helpful as any formal, non-fiction history books about the period (which I am anyway not qualified to recommend). Wellknown writers in Dutch literature who wrote books dealing with this are Adriaan van Dis, Jeroen Brouwers, Rudy Kousbroek and Hella Haasse.

Some books that might be most relevant: Hella Haasse's Oeroeg has been translated into English (sometimes titled The Black Lake ) but does not deal with the camps directly. It is however an excellent read. Clara Olink Kelly's The Flamboya Tree: Memories of a Family's War Time Courage is a memoir about life in the Tjideng prison camp. Jeroen Brouwers' Sunken Red is a novel/memoir about life in the Tjideng camp and its repercusssions on the authors later life, though the writer has been accused of exaggerating the horrors somewhat.

Hope that helps!