From a comment by Sassa NF:
In real life I asked my parents why they would still go and vote - I asked this after USSR collapsed. The answer was exactly what I said - "are you mad? It would instantly be known and there would be consequences"
I became curious and asked my mother. She was born in early 1950s, so her reply covers the years 1969 to 1986. (In 1986 the Perestroika began and the Soviet regime started loosening up).
She said she first attended elections, out of curiosity, in 1969, when she had become able to vote. She hardly attended any elections afterwards, up to 1989. There were no repercussions. She was a usual Soviet person, with a university degree (an economist).
Her brother was a Communist Party member, and served in a district committee apparatus actually (a kind of sub-Mayor's Office apparatus responsible for governing a district in Yekaterinburg, then Sverdlovsk). Furthermore, it was part of his duties to ensure the presence of "propaganda materials" of various sorts on the streets, as far as I remember.
The only reprimands she got were reprimands by her mother, my grandma, who reproached her mildly for being so negligent while her brother was sitting through the day in a voting station (he was on an election commission). He never suffered due to his sister's absenteeism.
My father never voted either. He woke up early on the day of voting and went to the voting station to buy some of the deficit goods before they've been sold out. That was quite an incentive. A Western person would find it hard to imagine what an incentive it was in a deficit economy. After buying the goods, he made an about-turn and went home, having done sweet FA in terms of actual voting.
He told me that hypothetically one could be reprimanded at work for not voting, and he suspects that one could fail to get some of the better amenities available to those who fawned to the system. That is, you could fail to obtain a tourist tour to a country in Eastern Europe. But he did not consider that a serious thing. He was not a Party member, and belevied the Black Sea coast to be a-okay for a family trip, if in exchange you could shirk at least partly from participating in the circus.
What was the reason for such high turnout?
I tried to google for some texts written by professional historians on this topic. A quick googling brought up one small article, the author of which (Alexander Fokin) mentions the importance of propaganda and the factor of people being cautious not to fall out of line, not to attract attention by their absenteeism. But was that hard enough incentive in the post-Stalin years? I also found a dissertation by a Podosinnikov Andrei, covering the period from 1950-1970. It states that the elections were widely propagandized and turned into a kind of holiday for the majority of the population. In that holiday atmosphere, it was just keeping with a tradition to go and "vote", even though the majority of the people understood the phoniness of the process.
I am sure that in the Stalin era my parents would have attended the elections: the fear was strong while Stalin was alive. But I have no reason to doubt their accounts of the 1970s-80s period. One could suggest that there was a wide-ranging manipulation of statistics by low-level commissions. My perfunctory googling brought up no works on that count. If I found some in the future, I'd expand my answer. Until then, it'll remain basically a couple of retold first-hand accounts.