She's an actor. In her every day life she is probably quite shy and introverted. But she has the ability to put herself in another mindset, as an actor. Modelling in photo shoots is the same as acting. Again, to use a personal example, I'm an extrovert but I also tend to enjoy a lot of introverted activities. When I am on my own I am quiet, shy and reserved. If you get me in a group of people I'm a lot more bubbly and outgoing. If I'm just going to be running errands I will wear jeans and a hoody, if I'm going out I will wear something nicer and more showy.
Now, those examples are just ordinary people examples, and are things that Emma probably does as well. When you add in the fact that she literally plays pretend for a living, it's not that hard to push that to the next level and assume that it's easier for her to switch into the mindset of an actor and do what she needs to do. Even in her recent interview on Ellen, she said she was nervous to present an Oscar and Ellen said she couldn't tell, and Emma replied "well, I am an actress."
I already know someone is going to jump in and say something along the lines of "if Emma is acting when wearing sexy clothes, maybe she is acting that she's really down-to-earth and normal," to which I basically have to say that the evidence that Emma is fairly "normal" is a lot stronger. She's a super famous multimillionaire, so it's not fair to assume she's "normal" like you and I, but given her circumstances she seems very approachable and honest (were I in her position I'm not sure I'd be similar... I'd probably be quite the diva!). Again, a recent example is the fact that she and her makeup artist blatantly post how much makeup and other products she's using. So many celebrities wear loads of makeup and pretend they don't (and that's FINE! I LOVE makeup, and I love people who wear makeup!), but Emma is often seen looking fairly "normal", sometimes a bit grumpy AND she is open about using things like fake lashes, 3 different kinds of foundation, etc. These are all things I can relate with, despite my lack of fame and fortune, and they seem genuine.