What is the role of a witness during wedding?

I'd like to know what the role is of a witness during wedding ceremony besides signing the wedding paperwork? Is there a spiritual duty of some kind? A godparent or a confirmation sponsor has a role to be a good example of faith for the child they sponsor during the sacrament. What about the brides or grooms witnesses? Are they obliged to do something when the couple are bringing doubts into marriage? I was wondering if the witnesses should/could prevent remarriage of Christians who accept divorce? Should the witness prevent "former" catholic from remarrying by approaching their new ceremony and object to the first marriage? Are these witnesses also approached during the catholic annulment? I'm looking for the **Catholic perspective** for now, even though I understand that this role can reach beyond this denomination.