Let's start with the question: "What is a personality?"
Seems simple, but it is a good place to begin.
Personality is not a monolith, it is the combination of all the descriptors used to describe an individuals behavior.
So, pick some descriptors. In this, we will make an example: Briar is bubbly, energetic, and sincere. They have a lot of energy, they tend to be positive, and they state their feelings openly. Of these, let's call some of these descriptors "temperment" and some descriptors "values"
TBH, I already like this character. Sincerity speaks to me deeply as someone who struggles with it.
Because this is a TTRPG blog, let's dive into this more. Lets begin with temperment. Why is this person bubbly and sincere?
Energy, and positive moods are strongly tied to your physical and emotional state. A person who has their social, and physical needs being met is more likely to be happy and energetic. You can play around with this, for example, a person who is meeting social needs but not physical needs may be happy but not energetic. A person who is meeting health needs may be grouchy, but determined. Someone with neither will be low energy, and pretty miserable.
So for our example, we picked bubbly. This person is socially and physically fulfilled. They are doing something they love, they have close friends, they experience love. They are also getting all their basic needs met, and are uninjured. For a TTRPG character, this means the person is perfectly comfortable on the road with their adventuring party. Their interactions with the party let us know more about them. I.e. if some of their companions are grouchy, Briar won't take their barbs seriously.
But, DBS, I hear you say, these are all very conditional. What happens when Briar doesn't fulfill their needs? What if someone says something that gets to them?
Yeah. Exactly! That's called an interesting character! So. What are some triggers that can change your characters personality? Maybe just losing sleep is enough. Maybe they are resilient until someone in the party betrays them. Maybe the adventure has taken a turn, and they don't feel their purpose anymore.
A word of warning though; if you struggle with negativity in your life, you will definitely need to set triggers to bring your character out of a funk. You can get stuck with a negative character.
OK. Enough about moods and energy. Let's talk about values.
In our example, Briar is sincere. Being open and honest with your emotions and feelings is a value. This is something your character has developed from life experience. These will also be more resilient than mood and energy, and will not change.
These should come up in the backstory, and can be brought on by positive or megative influences. Briar may value sincerity if a lack of sincerity hurt them (either Briars or someone close to Briar), or if they really admire someone sincere who made a big impact. It could be both. In my experience, a negative experience will lead to a trial adoption period. During this period, the value may be kept, or discarded.
As an example, Briar had a best friend. They were close, but for some reason, they were unable to express their deep feelings. Something happens, and if Briar and their friend had been open with each other, tragedy could be avoided. Briar vows then to be sincere. The vow gives it the staying power to last. Briar is now sincere. If they ever start hiding their feelings, they will be reminded of this friend.
As mentioned, values don't shift as easily as temperament, but they do have their own dynamics. What happens if their values cause problems? What happens if your DM has some fun with your backstory and suddenly the events that shaped those values aren't what they seemed? How has the characters experience since those events shaped their life? How important are those values? Do those values cause your character pain?
Example: Briar is open with their feelings, and that vulnerability has led to betrayal. Are they going to be more reserved? Are they going to revert to using deflections to avoid vulnerability? If so, will this change be temporary? What brings them back?
Apologies if this is a bit rambling. Hopefully this helps some of you.