Well, I may not be a movie star per say but I'm an actress who lives in Hollywood! (Or thereabouts, won't get too specific here.) That kind of makes me a Hollywood actress, right? Lol.

I am not a lip curler. I don't know which face shapes you have observed cause lip curling, but I have a pretty small, pointed chin and lips that aren't particularly full. Naturally, I am actually the dreaded "smiley crier," even when sad I make a face that resembles smiling while I cry.

Anyway, your theory is very correct when it comes to theater/play acting. On stage, you can't "lose it" too badly or the audience will not be able to understand you through your sobs. This goes double for musicals. So you need to hold back, breath carefully, and keep careful control of your voice. This can result in the kind of sobbing you described.

My experience is that screen acting is a lot more lenient here. However, some directors can be annoyingly particular about everything: the way you enunciate each word, the way you move your body, and yes, even the way you cry. I have been told to sob or make faces in a way that is not natural to me. I do not like directors like this because I feel like they don't let me do my job. Like, I might cry slightly differently as a certain character than I would as myself IRL, but I think that is up to me to decide as the actress. And some things, like lip curling or husky-sounding sobs, are so unnatural to my body and my way of expressing emotion that having to do them takes me out of character and messes me up. But maybe this is something that I need to work on, too.

My experience is that most actors in larger parts/higher quality productions are showing real emotion. The degree of that can vary, but there is always something real there.

Just my two cents. I got cast in something new this summer and there is a monologue in which my character isn't directed to cry but is then described as "wiping her eyes" afterward. I'm already thinking about how to do it. I will probably just turn the tears on without asking, lol.

@Super-Secret:

Thanks for the info! Some more questions for you, then. Are your crying expressions on set usually similar to your crying in real life? By this I mean the type of crying you're doing. For example, if you're sobbing in a scene, would it be close to your sobbing in real life? Or is it usually custom made for the scene? Most people don't notice the "lip curl" thing, but have you come across actors who lip curl on cue AND lip curl in real life? 

Based on your experience, if an actor was to cry for real in a scene, how might it go? And do they need a few minutes afterwards to switch off? Finally, are you aware any ways to tell the difference between real and fake crying? I'm guessing this is difficult. I've actually seen (on TV) an actress who cried quite convincingly, but it turned out she was using those tear inducer things as a short cut.