Practically, for the repair itself, since technique accounts for a lot of repeat failures too.
Open the crack out rather than filling the visible line. Rake it out to a slight V so the filler has a key and some depth. Filling a hairline gives you a two millimetre skin with nothing holding it, which fails the first time anything moves.
Remove all the loose material and dust, then dampen the edges before filling. Dry old plaster sucks the water out of the filler immediately and it cures badly.
Fill in two passes, slightly proud, and cut back rather than trying to get it flush in one go.
Use scrim tape across the line if it has failed repeatedly. Bedding a strip of mesh in and skimming over spreads any movement across a wider area instead of concentrating it.
Let it dry properly before painting. Weeks rather than days for anything deep, and a mist coat first.
And a realistic expectation: in an old house some cracks come back regardless. If it is stable in width and only a cosmetic annoyance, a flexible filler and a repaint every few years is a legitimate answer rather than a failure. Not everything in a hundred year old wall is meant to be permanently solved.