Whatever I've Done

The moment James hears that Regulus is missing, it's as if his mind detaches from his body. He disconnects. Something within him breaks. It's happened before. It's not too much of a surprise.

The next thing James knows, he's standing on a battlefield with Voldemort's dead body in front of him. This is more surprising.

This is what happened while James was gone. James won't remember it, but he'll have to live with it when he hears the story.

(or)

James Potter has an evil alter ego which will do anything to make sure Regulus is safe.