[Guinevere]

Come to think of it, this encounter in the book is in quite a different setting - it is much more important than the one in the movie!
In the book scene Grond has just broken the great gate of Minas Tirith and no enemy has yet entered the City. Gandalf on Shadowfax alone stands there to hinder the Witchking from entering.("all fled before his face")
And when the horns are heard, it is indeed the Rohirrim who save Minas Tirith from being invaded.

In the Movie encounter, the enemy is already in the City.( PJ wanted some more fighting!) The arrival of the Witchking on the fell beast is impressive, but not all that important. Gandalf just standing unmoving and steadfast wouldn't make that much sense, since he's not guarding the gate against the invasion. Even if Gandalf would defeat the WK, what would it help the City?
Even the arrival and the brave fight of the Rohirrim aren't that important either. In the end it's the ghastly green termite-army of the undead who saves the town.

A lot of changes which in turn caused other changes ... (most of them not to my liking, since I am not at all fond of battle and fight scenes. I usually skip them when watching the DVD!)