Say for example we have a

<ww:checkboxlist
      name="selectedOptions"
      list="options"
      listKey="id"
      listValue="name" />

If we like to populate options upon validation failure, we could have a prepare interceptor above the validation interceptor in the interceptor stack.

....
   public void prepare() throws Exception {
      // populate the options property list with options
      // that are supposed to be checked.
   }
   ....