I watched the video of John repairing cracked and loose plaster, but my problem isn't that drastic but it is persistent.

Have several hairline cracks in various walls in a post war Cape Cod. I watched my father fight with cracks in about the same locations in the house for about 60 years. Probably 90 percent of the cracks are more or less vertical. Plaster doesn't feel loose along any of the cracks, and generally both sides of the crack are flush with each other.

Should I open the cracks up more? Like make them into "V" shaped cut in the plaster, and then use joint compound? Or is this totally the wrong approach?

Any suggestions are appreciated.