If you were your character, in this situation, how would you feel? Honesty lends credibility to unbelievable situations.
Think about how you would feel if your sister disappeared. How would you feel if your brother was running for mayor, but you knew he wasn’t the right man for the job? Put yourself in your character’s shoes and track those emotions for a while. Jot them down so you can see what fits your character later on, but give it some time first. Really daydream about how you would react. Of course, don’t create your characters as yourself, but doing this will add a layer of believability and genuine emotion to them.
The flip side of this whole “give your characters opinions” thing is that your characters are going to disagree with each other. Mary likes olives. Claudia does not. Father thinks John should support his brother’s campaign regardless of political differences, because they are family. John can’t support him in good conscience, brother or not. Adding real-life texture to your characters through preferences and opinions and disagreements will deepen your characters. It’s also going to make their world more complex- small conflicts, things to enjoy, preferences people surround themselves with that start arguments or create inside jokes. And of course, all of this is going to complicate the main conflict. The good guys aren’t all agreeing on what to do. Not all of them are 100% good. Rivalry between the bad guys means things don’t go as planned.
So think about what you prefer, what things you argue with others over, what conflicts you have with your friends and family. Listen to the opinions that crop up that guide people’s lives. Work bits of those things into your characters, and they’ll be more active, more complex, and more enjoyable.