Fixing plasterboard cracks

[cracks2020]

These pictures are of the underside of the top of some bedroom windows. As you can see, there are cracks where the horizontal meets the vertical, but I have a couple of questions.

1) Why do the cracks suddenly go diagonal as they head towards the room?

2) what's the best way to fix this?

Thanks!

 

[motorbiking]

If you aren’t a confident plasterer you can get readimix at toolstation or similar or just use polifiller. Apply with a clean plaster knife and sand when dry.

it looks like heat expansion in the ceiling joists .