I would speak individually with the role “creeper” but first I’d check in with the person who’s role is being overtaken. Why are they allowing that to happen? If they have a choice, would they choose to fully own their assigned role? If so, you may be able to help by using a team meeting to review Project Team and/or Meeting Roles and to make sure everyone is in agreement on what’s involved in each role. There is a chance that it’s okay to swap roles, but either way, the issue should be surfaced in order to clarify.