Sandbox games are always about creating your own fun. It's why I consider GTA IV the worst in the series - it removed all of the options to create your OWN fun, and relied only on structured missions. For what it's worth, though, Liberty City was beautifully created and a real technical marvel.
GTA V, however, appears to be returning to the SA mindset, giving the players tools and letting them create fun on their own terms however they so chose. It's still the best Sandbox I've ever played simply because of the amount of insanity that's stuffed inside.
The characters, though, are apart of Rockstar's missions, and so don't really come under the "sandbox" part of the game.
As long as there's more to do than errands and phone calls, it'll be a lot better than IV. Whether or not it lives up to the kind of freedom you want, however, is entirely subjective.