Scene stealing NPC's

[NotPrior]

Weirdly I've had nothing but good experiences with major NPCs in this manner, whether running games with them or having them tag along. The party I'm a player in is accompanied by a gremlin bandit we fell in love with and recruited, and was recently (for a brief adventure) helped by one of the PCs' absolutely busted OP grandad. I've also played an NPC companion in a game I ran which went really well- the players got genuinely sad about the NPC's pain and death, even though he couldn't permanently die while one of the PCs stayed alive.

It feels like the golden rule in all of this is that the NPC is not one of the main characters. To compare it to something I'd hope we're all familiar with, Minrah is a pretty good NPC follower and about as PC-like as one can be. NPCs can be servants, aides, friends, family, lovers, whatever, and they can be stronger then the PCs if they must be, but they absolutely are not the protagonists. They don't come up with the plans, they facilitate them. They don't lead the party, they remind them what they were probably meant to do and let them take it from there. They don't kill the boss, they take care of the mooks. Even if they hold off a different fire giant with each hand they don't fight the lich, and they certainly don't defeat him.