Haven't used MM fields but if you are trying to filter for only projects with a specific role/individual assigned to a task you might be able to achieve that with the following filter:
tasks:roleID={roleID}
tasks:roleID_Mod=in
OR
tasks:assignedToID={userID}
tasks:assignedToID_Mod=in
If you're just trying to add a column with showing Role/assigned users for only some particular values the following works too:
valueexpression=IF({assignedTo}.{role}.{name}="Copywriter",{assignedTo}.{name},"")
You can also use {assignedToID},{assignedTo}.{roleID} for the expression.
Although you might want to add a unique identifier for specific tasks you are looking for or the column would just show all names/roles that match the criteria.
Hopefully this helps.
Best,
Surya