The Ditz: In the first few series particularly, but starting in Series D he's been getting more wins and second-place finishes. He slipped back into this a bit in Series H (his first with no wins since Series C), but recovered in Series I.
Hidden Depths: For all of Jo's crude humour she is actually well clued up on a number of subjects.
Iron Lady: Usually the token female panellist yet is able to put the men down quite easily and the audience love her.
Insufferable Genius / Know-Nothing Know-It-All: He ping-pongs between these two tropes so often it's difficult to tell whether he's got his facts straight on something or not. (Probably the safest policy on anything he says is to not trust it unless Stephen specifically confirms that it is correct.)
Running Gag: Kestrels, dating from his very first appearance ("Advent"). John Sessions picked up on it in the series D episode "Drinks".
Token Romance: Phill has gone on record (in the making-of special) that his role on the show is to flirt with Stephen as much as possible. This may be partly a joke, but he actually does do this to some extent.
Berserk Button: Whenever a "Genius" person in the panel's there, Sean's sure to mock that person endlessly.
Epic Fail: Currently holds the record for the lowest score by a guest panellist on the show: -76 in the "Germany" episode. (Alan, the lone regular panellist, has done worse; -84 on Cleve Crudgington under normal circumstances, and there have been special circumstances where he's gone even lower.)
Too Clever by Half: Despite this, he tends to fall into forfeit traps, and has managed only one win in his appearances to date (two if you count his appearance in series J: he was the highest-scoring panellist that episode, but Stephen Fry "won" that show).
When He Smiles: It is very difficult to get Jack to do this or burst out laughing on television (radio is different, but you can't see him in that case unless you go to the recording). If you have got this reaction, you have said something extremely funny.
Fake Brit: Well, yes and no. Is actually Danish, but speaks with noticeable and posh-sounding Received Pronunciation, as she was educated in England and has long since become a naturalised British citizen.