Dialogue and second character's actions, reactions, pauses, tics, facial expressions and the like... but definitely through dialogue for me.
Just a quick example...
Character 2: "Are you sure about that?"
POV: "I was... ah, you don't think so?"
Character 2: "Sure, sure, if that's your take." Leon nodded and smiled, but Ryan knew him well and saw right through it.
POV: "Okay, well, I haven't made up my mind yet, so--"
Character 2: "You do you, man," Leon said in a rush as he dropped his gaze.
POV: A sigh quietly slid from Ryan's mouth. It was clear his friend thought him crazy for even considering taking Thomas back. Was he angry? It irked him to think he was, so he challenged his poorly concealed attitude. "Why do you hate him so damn much?"
Character 2: Leon's eyebrows rose, and he looked sheepish, but stubborn too, and yeah, at the least, miffed about something. "I don't... hate him exactly, and it's none of my business anyway... it's not my life."
POV: Yeah, he sounded defensive now, and that never went well. Ryan cooled his jets, not wanting to poke and prod.
Character 2: "Listen, I gotta go. I promised my mother I would...." His voice trailed off as he turned away and started walking up his street.
POV: Ryan sighed again, louder this time. Leon was definitely lying--he wasn't fine with anything--and he was going to do what he always did when things got uncomfortable. No doubt he wouldn't see him for a week. He watched his friend's hunched shoulders and hurried gait for a minute, but he never turned around once, like he would have done any other time. Annoyed at how their talk had played out, he strode away in the other direction. He was confused, but something about Leon's demeanor niggled at him.
Just an over the top example to illustrate how much a character can reveal himself. Hope this helps. Cheers!