YMMV, but I generall find that it's enough to unbold the rear mudguard stays from the frame. That's usually enough to let the wheel-less bike fit across the back seat, at least with bendy plastic mudguards, optionally lashing the stays to theframe a little further along with a rubber band.
Getting it into the boot space of a small hatchback can require turning the bars sideways, or even taking them off, to dangle. That way it's not even a 10 minute job to put back together.