MC discovers a personal ability/magic that he had never even heard of existing before: the ability to project his mind into the mind of a character in any story he reads. It is the ultimate in reality escapism. He literally becomes the character for the story but without any ability to change the storyline or progression he has to be really careful which characters he chooses to Tag-a-long with.
Identify with one (or more) characters, but not having any ability to change the story line or progression.
That sounds a lot like the way reading fiction is supposed to work.
If the MC can't change things, what effect can projecting his mind into the "mind" of a character have on anything?
He is not just reading the story anymore... he experiences it in the mind and body of his chosen character.
The idea for the story is not how his ability can change the stories (because they can't) it is how his ability is discovered, how his thoughts and feelings change and develop over time in response to his experiencing the "actual" experiences of the characters he tags along with.
Tell me you wouldn't change some if you were reading say Jurassic Park and suddenly found yourself in the mind and body of Dr. Alan Grant with no control over what happens...just along for the ride as he has to deal with rampaging dinosaurs. Or trapped in the mind of Frodo Baggins as he is pursued across Middle Earth by the Ring Wraiths. Or maybe trapped in the mind of Christian Grey as he dominates and seduces Anastasia Steele. Or maybe even trapped in the mind of Anastasia Steele AS she is being dominated.
How would YOU react mentally, emotionally, etc? How would the various experiences change you?
He is not just reading the story anymore... he experiences it in the mind and body of his chosen character.
Seems like this is what we used to call "imagination", something that most people developed in early childhood.
Perhaps that's no longer true, maybe people are now only just "reading words". Some studies suggest that is exactly the case - a decline in imagination, creativity, and empathy.
Books should affect the way we think about things, but a story about someone who is "trapped" and "along for the ride" with no way to get off sounds more like the diary of a mental patient who cannot differentiate fiction from reality, and who should indeed be "really careful which characters he chooses to tag along with".
Would be not unlike some kind of "possession" - maybe a good horror story.
As for how I would react - I watched Dr. Zhivago, and cancelled any plans to visit Russia in winter. So there's that...
But I also read Silence of the Lambs, without developing a taste for julienne of Julie, washed down with a glass of Brandy.
And I saw Jurassic Park twice, and I'm not in the least bit scared of dinosaurs...
(except for the ones who drive cars with Florida license plates)
And your argument is EXACTLY the idea behind the story prompt: no matter how good our imagination is, it isn't the same as experiencing an event in person (so to speak in this case as it is a mental tag along, the MC's physical body is not at risk). Experiences CAN change us mentally, emotionally, and even physically (as in bodily reactions to stimuli) all of which can also influence our behavior.
Your level of imagination while reading a story versus actually experiencing the events would be like a totally dry person (no imagination) vs a person that uses a spritzing bottle to mist themselves (decent imagination) vs a person that ducks their head under a faucet for a few seconds to wet their hair (good imagination) vs being knocked overboard from a boat in stormy seas (experiencing events). In all of them (except the no imagination) the person gets wet but the amount of wetness of the person and the impact of the cause of the wetness is vastly different.
MC's physical body is not at risk
What if the MC's physical body is not at risk due to the events in the story, but potentially could be put at risk due to events occurring where their physical body is located.
True...also from the body's physiological reactions to the mental stimulus created by the tag along.
Possibilities include heart attack, stroke, dopamine/adrenaline addictions, palpitations, seizures, ulcers, priapism ;-) ,etc...