The extra was the guy who appeared in Season 3 Episode 2, when Blair was walking in the school, he's walking down the stairs ( just to make it more reliable)
so, they asked him to join filming again for 3x21
here's what he said
Thank you everyone, I'm so glad to be in Gossip Girl again. So this is the 21th episode, which is also the second last episode, so the team's being very secretive about it. They film it by dividing into parts, makes it really hard to figure what its about. But these are the things i saw"
1) CB are visiting the doctor (therapy), Blair is still mad that Chuck is furious that she slept with Jack to get the hotel back
2) the doctor seems to like Rufus
3) They're attending an evening party
4) Serena's Dad appeared and is dancing with Lily.
5) SERENATE LOOKS LIKE THEY'RE STILL TOGETHER
Im not really sure about the above, but mostly accurate
he also took some pictures, but i couldnt get access to them, seems to be exclusively for the forum (which i lost my member password)
here's the source, anyway
but its in chinese, he's a chinese living in US