German sie vs Sie

[tiia]

In the situation mentioned with a person stabbing you, but still using the formal you, it is even recommended to use it. I don't know that movie in particular, but when someone on the street is approaching you and you want to make clear that the person is crossing some border, then use Sie. First, it keeps distance, second it makes clear to the people around you that you do not know each other (well), and in the last consequence: that you need help.

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The first name + Sie can also occur, if a group of people is using du, except one person, like the boss or so. This person might not remember the surnames, because everyone calls the rest by their first names.
At university these situations were not too rare. Some people were addressing each other with the first name and also mentioned the person only by their first name when talking about them to others. The only person that was always addressed as Sie+surname was the professor.

If you once use a certain formality you do not change it too fast. Example: I had two advisors for my thesis. With one I used du, with the other Sie. Because they were now working at the same place they used du towards each other. (I'm quite certain they had been using Sie at some point before.) Now imagine me writing emails to both of them. You will understand, that this causes some kind of conflict.
My solution: avoid addressing them personally at all costs when the email was sent to both.

This was a lot easier in Finland: At university everyone always used their first name and the informal you, including the professors.