And I am portraying that imaginary person as realistically (for the setting) as I can. There's no reason they need to think in terms of their story any more than I do in real life.
But all "realism" means here is "as you would imagine they might act".
If I decide that they would act differently, that's not a reduction in realism.
The last time I played my PC Thurgon in Burning Wheel, I (as Thurgon) returned to my ancestral homeland, Auxol. As part of PC build, I had paid for a loving relationship with my mother Xanthippe. Therefore, as per the rules of the game, when I told the GM that I go to visit my mother (whom, as per my backstory, I had not seen for 5 years), the GM said yes: unless the fiction clearly precludes it, a PC is always able to meet someone with whom they have a loving relationship.
Here's how it unfolded, as per my actual play report written at the time:
My PC is Thurgon, a warrior cleric type (heavy armour, Faithful to the Lord of Battle, Last Knight of the Iron Tower, etc). His companion is Aramina, a sorcerer. His ancestral estate, which he has not visited for 5 years, is Auxol.
At the start of the session, Thurgon had the following four Beliefs - The Lord of Battle will lead me to glory; I am a Knight of the Iron Tower, and by devotion and example I will lead the righteous to glorious victory; Harm and infamy will befall Auxol no more!; Aramina will need my protection - and three Instincts - When entering battle, always speak a prayer to the Lord of Battle; If an innocent is threatened, interpose myself; When camping, always ensure that the campfire is burning.
<snip>
The characters continued on, and soon arrived at Auxol,. The GM narrated the estate still being worked, but looking somewhat run-down compared to Thrugon's memories of it. An old, bowed woman greeted us - Xanthippe, looking much more than her 61 years. She welcomed Thurgon back, but chided him for having been away. And asked him not to leave again. The GM was getting ready to force a Duel of Wits on the point - ie that Thurgon should not leave again - when I tried a different approach. I'd already made a point of Thurgon having his arms on clear display as he rode through the countryside and the estate; now he raised his mace and shield to the heavens, and called on the Lord of Battle to bring strength back to his mother so that Auxol might be restored to its former greatness. This was a prayer for a Minor Miracle, obstacle 5. Thurgon has Faith 5 and I burned his last point of Persona to take it to 6 dice (the significance of this being that, without 1 Persona, you can't stop the effect of a mortal wound should one be suffered). With 6s being open-ended (ie auto-rolls), the expected success rate is 3/5, so that's 3.6 successes there. And I had a Fate point to reroll one failure, for an overall expected 4-ish successes. Against an obstacle of 5.
As it turned out, I finished up with 7 successes. So a beam of light shot down from the sky, and Xanthippe straightened up and greeted Thurgon again, but this time with vigour and readiness to restore Auxol. The GM accepted my proposition that this played out Thurgon's Belief that Harm and infamy will befall Auxol no more! (earning a Persona point). His new Belief is Xanthippe and I will liberate Auxol. He picked up a second Persona point for Embodiment ("Your roleplay (a performance or a decision) captures the mood of the table and drives the story onward").
Turning back to Aramina, I decided that this made an impact on her too: up until now she had been cynical and slightly bitter, but now she was genuinely inspired and determined: instead of never meeting the gaze of a stranger, her Instinct is to look strangers in the eyes and Assess. And rather than I don't need Thurgon's pity, her Belief is Thurgon and I will liberate Auxol. This earned a Persona point for Mouldbreaker ("If a situation brings your Beliefs, Instincts and Traits into conflict with a decision your PC must make, you play out your inner turmoil as you dramatically play against a Belief in a believable and engaging manner").
What is it "realistic" for Thurgon to do, when his mother chides him for having been away, and asks him not to leave again? Thurgon is a knight-errant of a holy order, so that is a big ask. But equally, he loves his mother, and is devoted to his ancestral heritage (as revealed by his Relationships and Beliefs).
And would he, realistically, make this a matter of his
faith? Especially as his Belief is that
The Lord of Battle will lead me to glory. I, the player, know that even if Thurgon gives it his all - by spending his last point of Persona - the odds of success are slim. Corresponding to that, Thurgon knows that his faith may not be strong enough for the Lord of Battle to answer his prayer.
I don't believe that there is anything that is
knowable here. The real question is "How do I imagine Thurgon acting here?" What makes sense, given the feel of things - things in the fiction, and things at the (virtual) table - at that moment of play? It's not a prediction - neither a prediction of what Thurgon "would" do, nor a prediction of what will make for a "good story". It's a type of intuitive response.
I made my call, and as it turned out - due to the lucky roll - it played out as Thurgon hoped. If that test of faith had failed, what would have happened instead? I don't know what the GM had in mind, but some sort of pulling down of Thurgon for his hubris would make sense, and that would take him and his personality in quite a different direction.
It requires them to make decision about the use of meta resource, existence of which the character is not aware of, thus could not make that decision.
First, it's not obvious to me that the character is not aware of this "meta resource" - the character can dig deep, try hard, and exert their will, and that is one way of understanding the expenditure of the resource.
Second, and my main point: the MHRP approach requires less thinking about rules than the D&D approach, of setting target numbers, performing arithmetic, and comparing resulting values to a look-up.
This seems needlessly cumbersome.
Yet it is how D&D (or any other roll, add, compare to DC/difficulty/etc system) works.