They must act on His spoken words. They needed increased faith for increased forgiving. Hearing and acting on God's word will increase your faith, and increased faith will give you increased power to forgive. When you forgive by faith, you can also claim divine power to forget by faith. It is divine to forgive, but it is also divine to forget.
Forgiveness is a powerful force.
We can loose others with our forgiveness, or we can bind them with our unforgiveness. We can loose ourselves from torment by forgiving, or place ourselves in torment by not forgiving. To even begin to pay the debt, that man would have to sell his wife and children into slavery. But the King had compassion and loosed him by forgiving the debt. But then the roles in the story were reversed.