The Exact Moment Each Person Became Assimilated In The Thing

Fuchs has possibly the most brain scratching death in the entire movie, as while the team find his charred remains and assert that he's taken his own life by burning himself to death, at no point does anyone seem to consider the following point:

Why would he want to kill himself? He was seen working hard on a new experiment for the team to test for infections and despite doubting himself a lot, seemed to really want to work with the team rather than against it. It's much more reasonable to accept that Fuchs was instead, murdered by none other than Blair-Thing.

In order for Blair-Thing to build its ship it would have had to scavenge parts from the facility, and under the blanket of a power outage which we can assume it started, the creature moved through the Outpost. This attracted the attention of Fuchs who followed the unseen figure outside. Fuchs and Blair worked together a lot and are referenced in the film to be close friends, therefore seeing Blair pass by the door would indeed cause him to give chase.

Confronted outside, Blair likely turned on Fuchs, but instead of assimilating him, overpowered, killed, and burned the body. At this point assimilating would be too time-consuming and the space ship assembly would take precedence. Leaving the body for the others to find not only removes a potential hazard for Blair-Thing but also throws them off the scent as they try to rationalize his actions.