It's impossible to predict how much/little pain you will feel.
It varies from dentist to dentist and from tooth to tooth.
Generally, the deeper the filling, the more likely it is for you to feel pain.
However, if the anesthetic is sufficient, this would be minimal.
The injections often hurt. It depends on which part of the mouth you have them and how good the dentist is. I have read that pain from injections is partly down to the dentist rushing them. The faster the fluid is forced through you, the more discomfort.
Some people have a couple of Nurofen 30 minutes before a filling.
However, sadly I have had many fillings and none have been terrible.