Here's something I posted in the other thread. It occurred to me that this might be an interesting discussion.
How do you make a personality for your characters? Do you figure it out ahead of time, or begin as a blank slate and fill it in during play, or it there some other track?
That's how I challenge myself, with my stable of seemingly similar characters. I strive to make them memorably different from each other (with varied success). If I sat down at a table without saying which PC I was playing, almost everyone in my local club could tell you if it was the Warlock or the Bard from 3 minutes of RP.
I don't generally pre-define personalities for my characters, though - except maybe in a very vague kind of way (|grossed out by/fascinated by/jealous of/attempts to mimic| biological functions for instance). I prefer to play with them a couple of times and see what develops.
I'm still trying to hone in on the critical personality elements for a couple of them, but others leapt into my mind fully fleshed from adventure number one.
My own technique leaves me satisfied, but there are still times when it fails. My level 10 Cleric still hasn't really developed a unique personality of his own - he pretty much has the default "Big Mike" personality (of me, the player). I also have yet to really come up with anything interesting for my level 5 Warden. Here is what I've come up with for some of the others though, briefly:
12 - Crazy, WF Barbarian: Damn the torpedoes, full speed ahead. Enamoured with ocean-borne life ever since his run-in with Captain Leadbottom (although still largely ignorant of how sailing and ships actually work), Crazy mixes his nautical metaphors and sayings up, and charges into battle without thinking too far ahead. This personality developed very much as a response to in-game events, and was not planned ahead at all.
10 - Spanner; Wrench of Gond, WF Cleric: nothing has come up yet This guy reacts pretty much the way I myself would, were I a battle cleric (which I obviously am not, but you get what I mean lol).
10 - Word, WF Wizard: Fascinated with biological life. Runs with a Prestidigitation up 24/7 that allows him to smell and taste (no in-game effect of course). Loves to taste people's hair and fingers. Wishes desperately to one day become a "real boy". This personality was roughed out before play, but greatly amplified itself in-game.
7 - Harmonica, WF Bard: NSFW to the max (I have to be careful about the audience when I play this guy/gal). Two mouths, and s/he's not afraid to use them. "Fully functional". Not too picky about gender. I always try to work out situationally appropriate insults for Vicious Mockery. This is another one that developed during play.
7 - Null, WF Warlock: Very grossed out by all forms of biological life. Won't let anyone move through his square in combat (unless it is REALLY crucial). "Don't touch me" is his mantra. I have this awesome rant that I whip out occasionally, essentially demanding to know why milk is suitable to drink but other various biological fluids are not. This personality is almost the only one that I fleshed out before playing the character.
5 - Foundation, WF Warden: Again, nothing of note has really shaped this guy. He plays pretty much the same as my Cleric, RP-wise.
4 - Blokko, WF Fighter: Possibly my favourite of the bunch. He came out of the Candlekeep Creation Forge (all of my WF characters except the Windrise Ports one came out of the same forge in Candlekeep, hence the Baldur's Gate background for all of them) just as it was exploding. His body was put together incorrectly (torso backwards, arms upside down, face all mangled), and his personality is extremely simple. He tends to instantly and strongly befriend unsympathetic NPCs (such as the goblin Urk in AGLA1-5 Silver Lining, to whom the name "Orcslayer" was appended). His tactical sense in unimpaired, however, and he is a very strong Defender. The bad assembly bit was pre-assigned, but the rest of his personality developed during play again.