No doubt that a majority of casualties were on the death marches or working on railroads and bridges.
My dad and his friend used to actually drop food parcels to allied prisoners working on repairing bridges in his area. (He also dropped food to Japanese prisoners later in the war who worked on bridge repair. In fact, he said one of the scariest moments was when a Ghurka guard saw him drop food to a Japanese prisoner, and drew his Kukri in anger. My dad ran for miles to get away because he knew the Ghurka tradition was that once the Kukri was drawn, it could not go back into its scabbard without tasting blood).
The cruelty he witnessed seemed to be behind the wires, with executions and public torture by both sides being prevalent.
I do recall hearing stories from other people outside of my parents while growing up in South East Asia, but once again, the curation and editing of those stories rarely get past local areas. Nobody outside of the countries where it actually happened seems to want to hear, or repeat those events.
For instance, hardly anyone in the West seems to want to acknowledge how easily the Japanese walked into Malaysia, with almost no opposition. When they occupied Kuala Lumpur, they shot up the clock tower in the railway station near where my father lived, and that was it before the foot soldiers moved in. When the allies recaptured KL in later years, they carpet bombed the railway yards, killing hundreds of civilian labourers who lived in shanty towns within the yards. The tales of a triumphant US/English force recapturing their colony usually tends to omit details such as these.