Hello Goods, in mine, it was the external who did most of the talking - I'd say the question split was about 3/4 external, 1/4 internal. The internal had some questions but these mainly led from discussion with the external. The internal's job is to effectively chair the whole process, to explain the procedural elements to you and more importantly make sure you understand them. I've heard stories about harsh internals but I think that's more to do with how they were as individuals rather than what is officially expected of them in a viva.
With regard to revisions, yes internals have some influence, but it is usually a result of discussion with the external. In the case of minor corrections the internal does indeed sign them off, but if major revisions are required both of them need to see the thesis again.