No. But I suspect that they have some sort of a close correlation to the beliefs that the character has...
Beliefs in BW are (broadly speaking) ideals or goals that animate/motivate the PC. Here are the Beliefs for my PC Thurgon and his sidekick Aramina:
Thurgon
The Lord of Battle will lead me to glory
I am a Knight of the Iron Tower: by devotion and example I will lead the righteous to glorious victory
Xanthippe and I will liberate Auxol
Aramina will need my protection
Aramina
I'm not going to finish my career with no spellbooks and an empty purse! - next, some coins!
Thurgon and I will liberate Auxol
If in doubt, burn it!
Thurgon has 4 Beliefs because he has a Trait,
Sworn to the Order, that permits an extra order-related Belief ie the one about being a Knight of the Iron Tower. Because Thurgon has the
Faithful trait, one of his Beliefs must be about that - the one about the Lord of Battle - and if he ceases to have such a Belief he will also cease to have the Faithful trait.
Beside their connection to particular elements of PC build as just described, Beliefs play two main roles in the game, one GM-facing and one player-facing. On the GM side, they establish the paramaters around which the GM is expected to frame situations. Following from that, they provide the parameters the GM is expected to have regard to in narrating failures and complications.
On the player side, Beliefs are a marker for what to have regard to in declaring actions for your PC. Broadly speaking, when you declare and then play out actions that engage your PCs Beliefs - whether by manifesting it in play, or driving hard towards fulfilling it, or actually fulfilling it, or finding yourself conflicted in relation to it - then you earn what the system calls Artha, which is (broadly) what you might call hero points. Like plot points in MHRP, these can be spent to manipulate dice rolls and hence increase chances of success. It also plays a modest but real role in PC advancement.
Generally a player can change Beliefs at will. The GM is allowed to make the player hold off if it looks like an attempt to squib or dodge an unfolding situation rather than actually confront it.
In the situation I mentioned, the player whose PC suffered the naga's Force of Will had to change one of his PC's Beliefs from whatever it had been (I can't remember) to
I will find Joachim for my master. Once Joachim was killed, the player changed the Belief - with my concurrence - to
I will bring Joachim's blood to my master.
This is undoubtedly a limit on the player's agency. I as GM am getting a say over aspects of the game - ie how PC goals/thmes/orientation-to-action are signalled - that normally is reserved to the player. But it does not in anyway limit the player's ability to characterise or portray or "pantomime" is PC. I know, I was there, he was still playing his socially incompetent mad-as-a-cut-snake shamanic snake-handler from the hills!
@prabe: this is also relevant to your post upthread - about the PCs'
internal life.
The effect of Force of Will on the player's internal life is spelled out in the spell description - I posted it upthread:
This spell allows the mage to implant forceful commands into the victim's mind. The words of the mage becomes thoughts - as if the victim had formulated them himself. This is a very powerful spell - the words of the sorcerer are permanently embedded and resonate against the character's personality for the rest of his days.
The mechanical effect on play is as I've described in this post: it has no effect on the player's ability to declare actions for his PC, nor on the way he characterises/pantomimes his PC.
Now, if a player's conception of agency in a RPG is my private imaginings about what my PC is feeling and thinking then yes, FoW is a burden on that: if you're playing sincerely you have to imagine your PC feeling the forceful commands of the dark naga, and the impulse of hunting first for Joachim and now for his blood.
But two things;
(1) As
@AbdulAlhazred has said upthread, this is no different from the GM telling you
you see a dead-end in front of you. Now, if you're playing sincerely, you have to imagine your PC seeing a wall.
(2) I find it odd that, in playing a RPG, I would treat
my private imaginings rather than
the content of the shared fiction as the focus of my desire for agency. Because playing a social game based around a shared fiction is necessarily going to constrain one's private imaginings.