Update all fields in a Word document when user types anything

Question

I want a way of updating all the fields on a document automatically. I currently have a macro which is linked to F9. This macro updates all the fields in the header and footer, as well as all the ones in the main document.

Sub UpdateFields()
    Dim oStory As Range
    For Each oStory In ActiveDocument.StoryRanges
        oStory.Fields.Update
        If oStory.StoryType <> wdMainTextStory Then
            While Not (oStory.NextStoryRange Is Nothing)
                Set oStory = oStory.NextStoryRange
                oStory.Fields.Update
            Wend
        End If
    Next oStory
    Set oStory = Nothing
End Sub

Apart from setting the macro on every key, how would I make it so this macro runs when the user types anything?

For example a user may place a field in the footer or header which shows the amount of characters. If this was the case I would like to be able to see the characters field update as I type.