My walls and ceiling have settling cracks.
We got them when we built an extension. We just used filler and painted over it and they didn't come back :)
what kind of filler? A putty?
A drywall compound or painters caulk.
Assuming they’re not growing, then drywall compound or painters caulk.
if they’re growing or moving, you’ve got structural issues and need a pro to help your figure out that first
No shrink drywall patching compound.
How to Repair a Ceiling Crack