Alan: *On Mac's ear piercing* Some people find piercings repugnant. Fortunately, I am completely unpierced.
Mac: Well, even I draw the line at piercing assholes. *walks away*
Alan: ''Exactly!'' *walks away too, stops, and does a double-take* You...YOU!
One episode has Joanna complaining that Alan is never spontaneous. Later, he calls Joanna's office and tells her to come to his office. Harriet enters instead, and Alan bursts out of a cupbaord and begins dancing around, in his underwear, playing the flute, while she stands there in shock. When I first saw this scene I couldn't breath for laughing.
Caroline taunting Guy into signing a form saying he didn't sleep with her;
Caroline: "Nnnope, definitely didn't know about the Sue White thing. Do now, though. What a lot of things I know! I'm a walking encyclopediaaa!"
In Season Two, when Jake takes Caroline home after their date, Guy is looking out at them through the window as they awkwardly talk while leading up to the "end of the date" kiss. In a fit of anger and jealous he opens up the Swiss Army Knife (that's attached to his keys) and throws it at Jake's head. Jake has absolutely no idea what's happening, except that he's in pain, and Guy can be seen running in front of the window throwing his arms up in glee.
When Alan comes up with the medical alphabet song to try and impress the director filming at the hospital. He fails, and Alan is seen later venting his frustration.
Alan: A is for aperectomy, B is for barium, C is for cystitis... F is for fucking director... S is for slut and tiny tits is T and V, W, X is for wanky-wank and Y-fronts is for Y and... oh, cock off!
Alan, getting angry at Mac and the surgeons in general, walks into the operating theatre and eats a gall bladder.
Caroline: Dr. Statham is eating the patient!
Joanna bends over Harriet, wearing an unflatteringly low v-neck blouse, while Harriet is about to start eating a banana, and starts ticking her off about some intricate bit of administration on the computer. While Joanna's attention is on the screen, Harriet looks at the deep 'v' and clearly gets the urge to put her banana into it... and does so. The following under-reaction of both characters had me almost screaming with laughter.
Martin and Guy discussing his love life:
Martin: (On Karen) She's mad for it! She's like a wild animal!
Guy: What, like a zebra?
Guy: A warthog?
Martin: Exactly! ( Guy pulls a "WTF?" face at the camera)
"I'll shoot your pointy face off!"
Martin eating Karen's phone, then later on while talking to Caroline at the pub, it starts ringing.
Guy singing to the tune of Art Garfunkel's Bright Eyes. During surgery. "You've got a ginger chiiiiiiild! Flowing out of your peeeeeeeeenis! Following the river of sperm downstream... or is it... cream?"
Mac and Caroline deciding on Geordie accents before a surgery, and Guy's attempts at it are appalling and hilarious. What's especially funny is that, when the scene is cut back to, it implies Caroline, Mac and the nurses have been doing surgery and discussing Martin's exam results the entire time. They only stop when Guy threatens to smother the patient.
Also from that scene, Guy lights a fag and when Caroline and Mac stare at him in horror; "What? Does it say no smoking?"
From the same episode, Joanna nailing Alan with a giant exercise ball, cutting him off mid-sentence when he tells the other women at the gym he's "not looking at their...leotards!"