Found over at ToRN:
He[John Noble]did, in fact, talk about two additional parts in the EE, and discussed the 'falling over' scene in quite some detail. He said that Denethor is yet again having a go at Faramir and that Faramir is essentially telling his father he is wrong (that must be the '.you would not know your son' bit) and that Denethor stands up in a rage (he did use the word 'make to strike him' at one point, but when I asked about this he seemed to change to it just being Denethor standing up in his rage and making more of that than the striking.hmm.) but then trips over his robes and falls over backwards. He's quite dazed, and when he looks up, he doesn't see Faramir, but he sees Boromir (this sounded quite nice, but I was still a bit miffed at the falling over *g*) and he says something to him, but then the image of Boromir fades and he sees Faramir again.
link to the whole article: