How do I best repair a crack in our plaster ceiling that changes slightly as the house shifts with the seasons? The house is 50 years old and the basement wall beneath this room has a crack following the cement block outline that widens and narrows with the seasons (at its widest I can put the tip of my finger in the crack but only part way). There has never been any water leaking in at the crack. But the room above it has a crack running maybe 5 feet long the length of the ceiling. Will a flexible poly filla and tape work and how do I do it?

You can't fill a crack that comes and goes due to seasonal changes. You have a bigger issue than a cracked wall, you have a failed foundation and need to have it professionally assessed by an architect or structural engineer. Houses are designed to flex.