Again, the DM is responsible for providing bonuses or penalties, even advantage of disadvantage of the circumstances call for it.
So, a halfling is generally considered more charismatic as a standard, but the DM might apply Disadvantage to all Charisma rolls if the halfling is trying to influence a starving ogre who sees her as food. The DM might even be more harsh and apply both Disadvantage AND a hard penalty of -1 or -2. It all depends on the DM and circumstances.
Guys. It's not meant to be a perfect science. It's meant to give a rough measurement so you can actually roll for the character as opposed to trying to roleplay. What I mean is, if I play a super Charismatic rogue who can talk anyone into just about anything, the ability bonus and proficiency makes it so I can persuade via a skill roll. I as a person might really suck at talking people into anything, so I couldn't roleplay my way out of a paper bag in such scenarios, but I don't need to because my character is Charismatic and proficient in such things. I can just roll and have the DM say, "You did it. You convinced him to give you all his money."
