Everyone is "charismatic" in different ways, using the skill system or the mechanics to represent this isn't going to turn out well, so I'd suggest some custom "feats" or "boons" assuming you actually want mechanics for it.
Since we're talking about movies, lets talk about Star Wars.
Han Solo wasn't a diplomat, but he was a smooth-talker. Charm your boots right off. Convince the king to spare their lives...probably not.
Leia on the other hand was a diplomat. Could convince the King that we're actually the good guys by simply saying we are. Charm your boots off? Probably not.
Old Ben (not to be confused with young Obi-Wan) was the Wise Voice. When he spoke, people listened. They didn't necessarily agree with him, or like what he had to say, but they would stop and listen all the same.
Luke was the Peppy Kid, (at least early on) and had an endless stream of optimism.
Like the party dynamic, the key is breaking down "Charisma" into its different elements and making each party member strong in (at least) one element but not all of them.
Quite honestly though, attempting to replicate movie and literature characters in D&D is a fools errand. Literary characters succeed because the plot demands it, they fail because the plot demands it. They're charismatic when its useful and they're uncharismatic when it's necessary. There's no random rolls here. To that end, as I suggested above, giving each character a "boon" might be the correct way to go and that when a situation calls for their aspect of charisma to be applied, they automatically succeed. This could be a tag applied to certain encounters or certain NPCs. The players would be unaware of course, and using their auto-skill when the situation doesn't call for it, or is explicitly against it, would turn out poorly for them. This would give the game a more "cinematic" feel by taking away the random element of success or failure from the dice. The players would succeed or fail because they matched (or didn't) "Unique Ability A" to the scene.