Even though Harry is my favorite actor from the cast, I think his acting is a little over the top in the sneak peek from next episode. I mean I know Magnus is dramatic, but he is sort of coming across as a caricature of who Magnus actually is in that scene.
You know, it’s interesting you point this out because I personally don’t believe this is a misstep on Harry’s part; I think this is a deliberate choice in terms of how Magnus behaves with various people. The reason I say this is because I’ve noticed it’s been a very consistent trait Harry has given Magnus when he’s around those he doesn’t fully trust or know.
I think in S1 and throughout most of 2x01, we see him largely around Alec, someone he’s willing to be more genuine with (Clary, too). But watch him in 1x04 (with Elias where he twirls like 3x in the scene, but also when he’s flirting with Alec while Jace is watching – when he flips that table with his foot, please), 1x07 where Jace and Luke are both present (Jace’s presence is usually the main factor because Magnus isn’t especially fond of Jace), 1x12 during the Institute meeting and when they’re going to find Ragnor (again, Jace (and Hodge) are present). Even the way he approaches Jace and Clary in 1x12 after the kiss is over, there’s a swagger you don’t see when his focus had been Alec.
You’re right, it’s an exaggeration and it’s not representative of who Magnus actually is. But I think that’s the point: it’s a deliberate front he puts up, a way of deflecting people from seeing the real him. Magnus does not like to be seen as who he really is. He’s an extremely guarded individual, and the hair, the makeup, the clothes, the gestures, even his speech patterns – that’s all there to shield him. The real Magnus is a privilege reserved for the rare few who have managed to sneak their way into his heart. At the time of that scene, Simon isn’t one of those people.