What is a possum thinking?

The stare is not defiance, it's optics. Brushtails have a tapetum lucidum - a mirror layer behind the retina that bounces light back through it for a second pass - which is why the eyes flare in a torch beam and why the animal appears to be locking eyes with you. It is mostly just dazzled.

The rooftop situation is a housing problem, not a personality one. A brushtail wants a tree hollow, and a hollow big enough takes a eucalypt somewhere north of a century to produce. Cut down the old trees, and a roof cavity is the next best thing on offer: dark, dry, defensible. They're also fiercely territorial about it, which is why it is always the same possum, and why evicting one just opens a vacancy.