"Personality" is basically the series of things that distinguish each individual. In literature this largely has to do with a character's means of expressing (or not expressing) themselves, such as their dialogue and their opinions. Real personality extends to things like clothing style as well, but in a book we may have to remember or be reminded of this, which is more tedious than simply letting the character speak their personality.
Some of the most direct personality-setting phrases are the ones in which the character is announcing their feelings. For example;
- "I feel", "I believe", "I think", "I should"
- "We must" "You ought to" "There should be"
Stories about a character's past are also insights into their personalities: "I remember when," or "When I was younger".
The most helpful individual words are probably adjectives, such as arrogant, hardworking, chatty, greedy.
However, personality in literature is often complex, and built out of subtleties; it is rare that you will find a character who is so straightforward as to essentially say "let me tell you about my personality". For example, consider this revealing passage from Beowulf:
More I hear, that the monster dire,
in his wanton mood, of weapons recks not;
hence shall I scorn—so Hygelac stay,
king of my kindred, kind to me!
Beowulf has heard that Grendel doesn't use weapons, so he vows not to use weapons either; by doing so, Hygelac, his king, will "stay kind" to him. From this simple statement we learn several things about Beowulf;
- He values military intelligence and uses it to inform his battle plans; he is a careful person.
- By rejecting the use of weapons, he shows that he is self-confident, sporting, and fearless all at once.
- By taking Hygelac's favor into consideration, he is making long-term plans and finding ways to extract the most from every opportunity. He is as much a businessman as a warrior.
- By announcing the reason for his actions, he makes himself accountable to the other men, indirectly addressing the suspicion that he is doing this just to be boastful. Beowulf is a sociable person who knows how others think, and he guards himself against treachery.
However I don't think any of the words or phrases in this passage could be directly identified as being any more or less responsible for conveying Beowulf's personality.