A model’s comfort during a shoot is essential to the shoot being a success. Certainly there are some models who are able to control their face enough to not portray discomfort (often most useful during poses that are awkward and/or painful), but not everyone is capable of doing so.
The bottom line, however, is that during a shoot, the model is responsible for maintining her comfort at a level that is satisfactory to her. That means, that if a model is uncomfortable in any way during a shoot, she needs effectively communicate that to the photographer using actual words, not body language, expressions, sighs or other not-so-obvious displays.
If her shoes are pinching her feet, and she needs to take a break and sit for a minute, she can’t rely on the photographer to tell that from her body language. She needs to say, “photographer, these shoes are killing my feet and I need to sit down for a sec. Can we maybe bring in a chair I can pose on, or try some poses on the ground?”
If the model’s really thirsty, and will faint if she doesn’t have a drink of water soon, she needs to say, “photographer, I’m parched and really starting to feel it. Why don’t we take this time to change the lights around for our second look while I get a drink and change?” She can’t expect the photographer to know how thirsty she is, or that she may faint because of it.
If the photographer’s over-complementary directing style is making the model uncomfortable, she needs to say, “photographer, I appreciate your feedback, but the constant, over-enthusiastic complements are making me a bit uncomfortable. Would you mind toning it down a bit? I’d appreciate it.” Some photographers think that models like being complemented, and in reality, some women are simply weirded out by frequent complements.
If, for whatever reason, the photographer doesn’t listen to the model and she continues to be uncomfortable, it’s up to her to say, “photographer, I’m not happy with how this is going, and I’m afraid it’s going to start showing in pictures, rendering them useless for both of us. Either this has to change, or I’m going to have to pack up and leave.” If the photographer doesn’t allow the model to sit, take a drink, or otherwise feel comfortable, then the model needs to hike up her britches and say, “You know what, this isn’t working for me anymore, and I need to leave” and then, leave.
Models, when shooting, you need to communicate with the photographer during the shoot, but especially if you’re not feeling well or uncomfortable at any point in time. If you don’t, it’s your own fault for letting the situation continue past the point where you’re uncomfortable, and it will ultimately be your fault for useless images, blisters, passing out because of low blood sugar, or whatever else may happen. Keep that in mind when shooting.