I used to just fill the gap and play the stereotypes, and used roleplaying to differentiate between Sword & Board human fighter #1 from Sword & Board human fighters #2, 3, and so forth. Over my decades in the hobby, I have become more focused on realizing PC concepts. Part of that is the influence of other games- mostly HERO- but also a certain boredom with playing the same PCs with minor variations.*
So since the 1990s, my first question to myself is “What would be fun to play in __________ setting?” And my answers have come from inspirations as diverse as literature, minis, specific game mechanics, music, obscure weapons, movies, TV series, comic books, art (genre-centric or otherwise) and even the odd dream or two.
Once I settle on a concept, then I try to figure out how I can model the character within the system and setting.
Just to be clear, I still play gap-fillers. But don’t expect me to play cookie-cutter ones...anymore. One of my last 3.5Ed PCs was a gap-filler. The group was playing through Return to the Temple of Elemental Evil, and two of the 3 players with any healing capacity moved away. The remaining character had 1 level of Cleric.
I retired my character and created a new healer PC. The inspiration was trifold: the original Sacred Healing feat, the Geomancer prestige class, and DC Comics’ Swamp Thing. The character took all the plant-centric Drift options from the Geomancer class. With a high Charisma and by taking Extra Turning to bolster his number of Turn Undead attempts/day, he was able to fuel Sacred Healing enough that he rarely had to actually cast a healing spell.
He couldn’t turn undead all that well (as a Cl/Sorc/Geo) but in that adventure, that was almost never a concern.
* “No, dude, this dwarf warrior has a GERMAN accent, not Scottish!”