(Once again, I am certain I am far from the first to mention this, but never mind. Here are my thoughts)
I’ve been thinking about the framing of shots in Sherlock a lot lately, then I got thinking about this scene. Look at that for a sec.
John’s hilarious reactions and the just plain bizzare sight of Sherlock kissing a woman aside, check out the framing of the shots with Sherlock and Janine. Two different angles, and in both of them Janine is blocking out John. She is almost perfectly filling the space where he would otherwise be in those shots (meaning that in a two-dimensional sense Sherlock’s line of sight is matched to where John is).
In the first, she is obstructing him and filling in his space (yes, double meanings) plus bonus horns over her head.
In the second, not only is she doing this but she’s kissing Sherlock as well. Meanwhile Sherlock has his eyes closed (even screwed shut) and is barely responding. He doesn’t tilt his head or move it forwards/backwards aside from the force of Janine’s kiss moving him, so his and John’s head’s remain aligned. If Janine wasn’t there, John’s outline would be touching Sherlock’s. If John were in focus there would almost be a forced perspective whereby they would appear to be face to face (much like the shot in TEH where we are led to believe Sherlock and Mycroft are playing chess).
You may say this is unintentional but framing is super important and almost always deliberate (this is true in all visual art forms). I mean John doesn’t need to be there really. He could do what I imaging most people would do if their best friend started making out with someone which is leave, go make a cup of tea, pretend to be immersed in a magazine or something. They could have shot this from the other way around so that the camera was pointing towards the stairs, leading on to Janine’s immediate departure. But no, they shot it with John obstructed, with the audience made aware that he is there, but we can’t see him. With Sherlock (who we are led to believe has just bathed in front of this woman and was fine with her sitting in his lap) making the bare minimum effort to respond to her kissing him while his best friend is in the room.
In summary, Janine is in John’s place, physically and metaphorically, and Sherlock knows it.