find out what their characters love, not want but love
now start threatening those things/people they love, little by little and then destroy the things/people they love
and watch them try to return the situation back to "normal" - trust me that'll be their characters growth.
Read Steven James book When Story Trumps Structure
He says don't think what will happen next but what goes wrong instead.
This is great advice for including plot hooks that will generate role playing. Doing this will create SUPER satisfying story arcs for those players, and you can weave them into your main plot as well.
For smaller role play moments I suggest looking at what the character's attitudes / usual actions are. Do they always ask about a npc playing music? Do they like to browse weird market stalls? Do they push for/against a particular organization in the world? (a church, noble, ideal, cuisine) What are their common role playing bits?
Give them more opportunities to indulge in those. Give them a minor decision to make. Paladin Bob always has something to say about dishonesty, give him a merchant who is obviously over-inflating his wares. Ranger Tiffany loves music, put a bard in the tavern that is terrible, or is weird (ala Jaskier from The Witcher). Sorcerer Gladys is disturbed by the way the local King is deified. Make the character that has information she needs be a supporter of the king's cult.
For players that aren't as active, give them some hooks too. Easy ones. Really easy, that can be resolved right away. There's a whole class of player out there that mostly wants to observe the game, not be super active. Throw them softballs, like a duelist fighter getting to comment on a duel that's happening. A cleric is asked for a blessing by a mother to be. A rogue with intimidation gets to quickly be a tough gal when some punk gives her the stink-eye.
