The one factor, at least based on a superficial reading of the parsha, that distinguishes a case of ones from a case of znus is whether the woman screamed out or not. "Al dvar asher lo tza'akah ba'ir" (22:24) -- the assumption is that if whatever happened took place in the city, the woman must be guilty as well or someone would have heard her screaming.
Ramban is bothered: why are we so hung up on the screaming? Maybe this woman lives in NY and knows no one is going to respond to her screams anyway, like the Kitty Genovese story, so she doesn't scream -- she obviously is not still guilty. The key question should be whether she was coerced or not, whether she engaged in an illicit act willfully, not whether she specifically screamed.
Ramban answers that you have to say that screaming is lav davka and the Torah is just describing a typical case.
Sefas Emes, however, says screaming makes all the difference in the world. Screaming is not just a siman, a way to raise an alarm, but screaming is the means to effect yeshu'a. "Al dvar asher lo tza'akah ba'ir" -- because had she screamed, she surely would have been heard and saved. Not seizing that opportunity is itself a crime.
(Without the Sefas Emes I think you have a bit of a pshat difficulty. The Torah tells us exactly what the man did wrong -- "al asher inah..." When it comes to the woman, if you read the pasuk like Ramban, it does not tell us anything. It just gives us a siman, "asher lo tza'akah," and leaves us to infer that she therefore consented and committed an immoral act. According to Sefas Emes, the failure to scream, the failure to avail oneself of the opportunity of rescue, is itself the crime. It's not just an inference but the Torah is telling us exactly what was done wrong by the man and by the woman.)
The Sefas Emes is not just a comment on this specific parsha, but is a comment on life. There is a lot of stuff that does not befit us that we come in contact with due to our having to work in and live in a secular society. What can you do -- ones! We can't so easily change when and where we live. The Sefas Emes is telling us that if it's really ones, then you should be screaming. If you passively sit back and do nothing, or worse, accommodate yourself to the situation or enjoy he situation, then all bets are off. If you want to be saved, you have to scream. And if you do scream, you will be saved.
(I had a very hard time trying to formulate this Sefas Emes. For some reason earlier in the week I became fixated on writing this point up and then I couldn't let it go. It became a mental block to my writing anything else. After mulling it over for 2 days I don't like the results. See S.E. in the Likutim, in 5634, and 5640 and see what you make of it.)