Every job comes with a contract. It's important that you understand that contract, and not agree to anything you're not OK with.
You are in a transaction with the organisation to sell your time and skills for a set price.
If you don't keep control on your responsibilities, you may find yourself being given more and more tasks, beyond the obligations of your contract, without being paid extra. Why work for free?
Any overtime should be paid. Any extra skills used should be compensated for. If you're doing more work, you deserve more pay.
The only possible exception may be if you fail to meet goals you agreed to. If the goals were fair, if you should have been able to complete the task, if the target did not change, then it may be understandable to ask you to perform more work to complete the task by a deadline.