Installing ‘TruStone’ veneer, directly on painted drywall

The keys are the "key". When real plaster is applied over wood lath, the base coat oozes through the gaps in the lath and sags or expands a bit forming "keys" that, when dry, hold the plaster layer to the lath.

This is a strong structure unless a substantial amount of the keys have been broken (from temperature or humidity swings, structural shifting, physical damage). When this happens in a specific area, that section of plaster is basically unsupported and is prone to cracking and buckling inward.

If the plaster is generally sound, you should have no problem. While drywall is more flexible and less prone to pulling away from the framing, sound plaster should be strong enough to hold the weight of a thin veneer.

If, however, significant sections of the plaster are bulging outward, or if there is a large amount of the surface coat pulling away from the base coat (a sign that the plaster is in poor condition overall), this is not a good basis for a top coat beyond paint or paper. In that case, you may need a more significant redo.

If the weak areas are just a few and not too large, there are specialized plaster washers that are used to screw the weakened section to the lath to prevent further damage. That may be sufficient if the areas are few and far between.

For those who came here from web search, I've done this and I can share my experience.

1. You install doors before.
2. There will be gaps between firebox bricks and door frame - cover them with cement.
3. Remember to raise doors high enough over existing opening to allow for the thickness of the new hearth stones (if you plan to put them) so that these little vents in the bottom of the doors have unobstructed air flow
4. If you plan to install stone veneer with corners build up a wood frame over bricks deep enough to accommodate these corner returns
5. Cover all the gaps with cement
6. If you built the wooden frame, you don't have any metal to put cement on. It's all covered with plywood and wire and cement layers.
7. If you had a mantle shelf you might need to redo/rethink how you want to secure it after the stone is laid, but before actually laying the stone.
8. Instructions call for cement type S ($7 per 40lb), but my experience shows that you need much more experience with stonework to use it than I had, so just use polymer modified tile thinset ($25 per 40lb)
9. Use cement type S + cement colorant for grouting
10. Forget finding the perfect stone, cut and cut and cut
11. Get yourself a vac enabled cover for angle grinder and use shop vac