One of the protagonists, played by actress Marina Alexandrova, wears upper and lower clear retainers in every single scene.
Here's what she had to say about this in an interview:

"I wore braces a while ago. And when they were taken off, I was given retainers. And before the casting, I decided to repeat the text - in the evening, with my retainers on. I started reading and I feel that there is an unusual pronunciation - a defect in sibilants. And then I realized: this is her grain! She has been struggling with speech impairment all her life. And she even chose a public profession in order to talk with students, overcoming complexes. And hence the struggle. And she asserts herself when she confronts the authorities. And so I came to audition, did my bit without retainers, and then I asked: "Can I take an acting mate?" And then I started doing the part in my retainers - my lip sticks out, my speech changes And I got the part!"

Lots of nice scenes:

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