How real should fiction to be?

In one of the groups I belong to on social media, a reading group, someone posed a question which although framed by the genre the group is interested in, applies to other fiction as well. This is the essence of what they said:

How real should you expect fiction to be? I recently read a book from a series; the early books I really enjoyed, but as the series progressed, the more far-fetched they have become. Obviously a writer of fiction has the freedom to create a totally imaginary world and needs to contrive various impossible or unrealistic twists to keep the audience engaged and entertained, however, in the book from the series I’m thinking about, what the main character does, and what happens to them is impossible. The physical injuries they receive would render anyone incapable of continuing to do anything at all, especially without medical intervention, and yet the action continues apace. This character then behaves in such, unethically and unprofessionally, that they would not be able in real life to continue. a way that he could not continue. “What was initially such a promising storyline just devolved into silliness…may sound odd, but feel very let down by this.”

I replied:

You make a really good point, and it’s so disappointing when that happens, that credibility is stretched beyond what a reader can accept. I had a similar experience with an American author, I absolutely loved; her characters, especially the main one was so engaging, so believable (even in the far-fetched fictional world) and then gradually her experiences became so wildly improbable – in a similar way in which your main character lost plausibility, that I began to be disappointed, then exasperated, and then annoyed, and then I stopped reading – halfway through a book! It was as you say, just silliness, and I too felt really let down, and disappointed. If the author admitted she was fed up with the character, she should have stepped away, stopped writing about her, and write something different.

Someone else made a good point – I believe, as a writer, the crime story should lie so well that the “truth” of the fiction doesn’t pull a reader out of it. Even as a reader, if I’m pulled from a story multiple times, I’ll give up on it. I know that feeling – when I start to mutter to myself ‘this is utterly ridiculous, for heaven’s sake!!  then I begin to think it’s time to move on to another book which doesn’t stretch the bounds of possibility to breaking point! Somebody else also made a good point – a totally realistic police investigation would be boring to read… it’s a case of finding a compromise between the two… Finally, this point sums up the balance between making what is fiction seem possible, keeping a reader’s interest and attention, and allowing unrealistic situations and characters to operate in a narrative which engages – there’s something wrong with the writing more generally if you notice that kind of thing.