I think you're missing the point here.
Who establishes that the character has that trait? Why is it established so?
This is what I mean when I say the
absoluteness of DM power in (some) D&D-alike games gets in the way. If the DM wants a specific situation to come about, they merely have to only
allow things into the fiction that only permit that, and don't permit other things. The act of "staying true to the established character trait" is not and cannot be protection from railroading because the railroading can be any that trait was established in the first place. And, at least in some cases, the DM may not even realize they're doing this.
Also, having the following things in sequential order...
...kinda shows exactly what
@pemerton is talking about. To Bedrock, this is so strange as to be almost alien. To Firebird, it's par for the course. If two people can understand the exact same thing to be weird to the point of almost inapplicable and common to the point of universal, surely you can both see how
just "guidelines" and "coaching" and "structured frameworks" might not be enough to achieve a particular end.