Dead Ringer started life as a third-person novel, until my agent suggested I make it first-person.
This was a big change, but it made the protagonists’ voices much clearer. The reader gets to sit in their heads, hear their thoughts.
This, I think, is the superpower of novels (versus TV or movies). You get to hear what the main character’s thinking. In a way, you even become that character while you’re reading.
I’ve used third-person for other projects since writing Dead Ringer, but the act of changing a whole novel to first-person taught me something. I learned to become much more attuned to the character’s thoughts. I began weaving internal monologue into third-person stories a lot more.
My advice: don’t get so caught up in action and dialogue and description that you forget the importance of communicating your protagonist’s thoughts.