There’s a tricky balance between revealing a character’s thoughts, pointing out the character is having thoughts but not revealing them, and not saying anything, hoping the readers know that characters have thoughts.
I spend a lot of time in FP POV, and other characters are the issue. With first person, you can usually talk about the narrator’s thoughts. It gets bogged down sometimes, but important thoughts are almost always fair game, and the lesser stuff can usually be glossed over. “I don’t know what I thought about that.” “My feelings on the matter were mixed.” “I tried to keep an open mind.” Those are all paraphrases for ‘the narrator was thinking but the author doesn’t want to go too far into it now.’
With other people it’s tricky, and it’s especially tricky for other people in a FP POV. If Alice, narrator, is talking to Bob, Alice is going to have a lot of problems figuring out what Bob is thinking. That’s real life, but it’s also really frustrating to read. Picking the right balance takes some finesse, and the author can’t get too bogged down in writing about Alice trying to figure out what Bob is thinking.
I’m rereading the first book of Shannara, the Sword of Shannara. It’s good, but not as good as I remember. The problem is that Terry Brooks goes way too deep into some people’s thoughts, and they don’t have anything interesting to say, and avoids others. He avoids Allanon’s thoughts to build drama, and that’s all well and good. But the fourth or fifth time Menion thinks something impatient and stupid, albeit with a little depth, I’m getting bored. It reads like early RA Salvatore. They were/are contemporaries, and they write like players going through a campaign.