A lot of this is aimed at the character creation stage, but if your current PCs are still low level and a bit aimless I think most or all of this could be applied to a fleshing out session to kind of refocus things with existing characters.

Now all the below is more what you'd call guidelines than actual rules, so sometimes it might not fit perfectly but anyway 'ere we go.

Characters should be written with at least a couple of NPC connections in their past. Family members, friends, rivals, whatever. If it's a rival/adversary it probably shouldn't something massive like The Demon Knight Who Murdered Everyone since that would overshadow the campaign but something more mundane like Gary from Pokémon. Or maybe if the character was an escaped slave the old slave trader, or if they were a merchant they rival across the street. Something that can be brought in for a minor plot if needed. If it's friends or family that's pretty self explanatory. This makes players actually think about how the character fits into the world and it gives the GM material to optionally slot in.

The characters should have some longer term motivation that is unique to them which runs parallel with being in the party. If they just want lots of gold, it just needs to be a step further as "I want lots of gold to do 'x'." That goal can be goal to get passage out of the region, or to build a giant statue of themselves, or to reverse transmute the gold into lead to smelt into a giant cannon ball to fire at the capital of their cultural rivals. Whatever it is, it gives a bit of insight into the character which is something hopefully the player keeps in mind.

Players should think about their alignments and backgrounds in how they interact with NPCs. I don't mean they should be rigidly guided by this to the extreme every single interaction (because that leads to a whole other set of issues), but sometimes it's good to think about alternate angles when things are kind of stuck. I see this especially when trying to negotiate with an evil or corrupt NPC and newer players just default to "Helping us is good and you should do a good thing!" arguments that are no sale, but if somebody remembered their character came from a shady background maybe they'd remember to bribe or blackmail instead.

Something I do for my own characters personally is write up at least one personality flaw. Hardheaded, untrusting, drinking problem, gambling problem, racist, whatever. Something that can come up and maybe change throughout a long campaign or maybe just never gets resolved. Again, important that this flaw is tempered against the flow of the adventure so it doesn't become a drag.

Similarly I write up one or two short and sweet physical flavor details. One character had the end of their pinky chopped off for thieving when they were younger. Another had an obliterated tattoo on their arm. One happened to get nosebleeds slightly more than normal. All of these things could be nothing or could be something. I put them on the sheet and left it in the hands of the GM if they wanted to explore these possible hooks. If it never came up again, it never came up again but at least it was there to latch onto.