Thread:Urkelbot666/@comment-26268104-20150826235822/@comment-25021327-20151003144106

Hhmmm... One way you could mark the transition would be to have him have one or more certain habits, or ways of doing things, and as the story progresses, he forgoes these habits. It could be something personal, like falling out of contact with friends or loved ones he is usually close with, and/or more professional, like how he goes about his work could get more determined, or haphazard depending on which way you go with it.

That was the first thing to come to my mind, just basic behavioural changes that show a shift in his personality from normal to obsessive.