You might have a spectrum, like "Rebellious/Obedient", "Hot-Headed/Cool-Headed", and so on. You might have numerical or descriptive scores in, for example, "Quiet", "Ruthless", "Bold", etc.

In short-form IF it can work to just pick one personality, for example choosing a single trait from a pool such as "Naive", "Sarcastic", "Impetuous", etc. I am not sure if that is the kind of thing you're envisioning when you say you're considering locking in the traits, or whether you're thinking of having several different traits that can be confirmed over the course of the game.

My advice for long-form IF is that it works better to have at least a few so that the MC's personality can be more detailed.

For instance, with multiple personality traits a player can play as someone who is high Ruthless, high Bold, and low Quiet (a loud, brash, ruthless person!) or low Ruthless, high Bold, and high Quiet (a soft-spoken, compassionate person who's keen to rush into action!), and many more combinations.

The same idea applies if you using personality spectrums. Having a few different traits means there is more room for an MC to express their personality in different ways, and for other characters to respond to it positively, negatively, with surprise, or any number of other ways. Just as with creating a single personality trait to choose from a pool, it also gives room for those moments where the MC is acting "out of character" or aligns with the expectations the player has set for their behaviour.

It is wise not to gate off options based on the personality the player has established - players like to have the choice to behave in ways that depart from how they have previously. It is also delightful when another character is shocked when a previously wallflower-ish MC suddenly shouts at someone in public, so I believe your thoughts about acting out of character are solid.

As for how traits get assigned, it's reasonable to have the player lock a trait early on with a choice such as:

I usually respond to public speaking with...

• Delight [Set Outgoing to Very High]
• Anticipation [Set Outgoing to High]
• A shrug [Set Outgoing to Mid]
• Nervousness [Set Outgoing to Low]
• Horror [Set Outgoing to Very Low]

Equally, you could track how Outgoing the MC is being over time and write flavour text or NPC responses to reflect that. This can sometimes feel less clear/visible to the player, so there may be a point down the line in which a character says "but you're usually so shy" and the player goes "eh? But I've been outgoing the last few choices, why isn't the game responding to that?". That issue can be mitigated by tracking particularly important Outgoing moments and thorough playtesting (and if using ChoiceScript, RandomTesting to check the spread of stats that players will be regularly hitting).

Broadly, I feel the former can feel a little more game-y with more distance between the player and MC, where the latter may close that distance. But both can be handled in perfectly effective ways, and it's really up to what feels intuitive to you.

There is no point in getting tangled up in one personality system or another if you find it hard to envisage what the MC is actually doing and saying. The numbers and personality descriptors are the machines driving the story under the hood, but what's important is how those personality traits affect the MC, their journey, and how they interact with the world around them.

Best of luck with your writing!