Why did Gandalf let go and fall off the cliff?

[Nar]

Mr Underhill:

McKellan did a good job of showing Gandalf's fear and humanity.

Agreed! McKellan's was an excellent performance all through. I did think the director meant to indicate Aragorn moving to help Gandalf, but not able to reach him in time, and Gandalf slipping as in the book. Reasonably enough for an action scene, the events were shown in the extended 'movie time' that movies always use in an action scene, where the same few seconds are shown from a couple of different angles. I didn't think the timing and editing quite conveyed that Aragorn was moving back but couldn't reach his friend in time and that Gandalf didn't have a chance to hold on-- he's shown free of the whip and clinging just a beat too long. However, it's not a major problem; I still enjoyed the scene. It was close enough.