I think for the practitioner, it truly would depend on their own boundaries.
children age 3 1/2 -4 and up know the difference between fantasy and reality.
it’s a cognitive “switch” that clicks in.
that’s not to say they get it right every time, or that there aren’t mental states where the line gets blurred, or where the local social structure artificially props up a fantasy element such as Santa Claus. But even those don’t usually last too much further.
within the blurred line category, you can also choose to believe something is real and reinforce it yourself by choice.
I believe in most cases though, displays of belief in fantasy when lacking social support tend more towards conscious acting where the line is very clear.
In that case, creating bad habits that transfers into non role play exchanges is at the same level as method acting issues perhaps and could be addressed that way.