Just knowing what you're inclined to do is a great step as a writer so nice work =)
I would work at this issue starting from your strengths. You said you do well with acts, "plot" I take it. So start with that. Think of the plot, get all the action planned out like you're comfortable doing. Then look at your characters who are carrying out this master plan. Why did one character do what he did? Did he have some motivation beyond problem solving? Perhaps some character trait or part of his personality made him react in such a way. Perhaps a past disappointment or tragedy. By thinking about these things, you're giving your character depth. Now start thinking about how to show this to the reader. Maybe you decided your character jumped into a battle because he is confident to a fault. What little tics, or mannerisms can you give this character to show his overconfidence? What's something that overconfident people to do in you, the author's, experience? Sprinkle those mannerisms into your narrative whenever you can.
Somebody told me once, always know more about your character than your reader. Have some facts or history about him in your head that you have never written down, maybe from his past. That kind of mindset will train you to think about character depth a lot more often.