Roleplay planning and hiccups

I'm not a particularly new DM (4 years) but recently tried to branch out into more roleplay-heavy games and pick up a fourth player only to find I absolutely suck. I can have complex NPCs with goals they want from the party and secrets on paper. and every single one sounds/acts like me just talking in a posh accent and not reacting well to my players actions or anything. Just really awkward, stiff acting and no mannerisms or even know what to say that doesn't sound like wooden exposition.

How do you plan/improve at the roleplay?

Also, I have four players. Two are hugely loud and love improv and the other two are shy and struggle immensely but still want to participate.

I don't know how to make sure everyone gets time without putting anyone on the spot and making the nervous ones too uncomfortable. Any advice would be great