Even though the paint was stirred at the store, you should always stir it again before you use it.
Use a broad, flat stick to stir the paint, as this will ensure the tint in the paint is dispersed properly so your colour will be accurate and consistent. When stirring the paint, make sure you use an up and down “scooping action” to ensure you are stirring the paint right through from the bottom to the top of the can. Do NOT try to stir the paint with a screwdriver or skinny stick - this will not mix the paint properly and will cause problems with your colour.
If you have poured your paint into a roller tray and small paint pot, and you paint for a while and then need more paint, stir the paint again before you add more to your tray and paint pot.