I believe the way roles are designed to work on approvals is that if there is a user with that role already on that project, when the approval is triggered, that user will be auto-assigned to the approval.
If that user is not already on the project, then yes someone would have to manually replace the role with a user.
More on that here: Create an approval process for work items
specifically the NOTE in the Approvers section of the table on that page.