My novel is in the first person, and it's tricky.
I know what's going on in the world around my main character, but that doesn't mean I can describe what I know is going on. I have to describe what's going on through his eyes, and as he sees it.
I started the novel in the third person, and then switched over midstream when I discovered that one character was in every scene. He's easily been the focus from the start, so it just seemed to make more sense. I don't know if that's common or uncommon, but that's the hand I've dealt myself.
Just because the first-person narrator of the story thinks somebody's a jerk and talks about him that way, doesn't necessarily mean that person is a jerk. But that jerk is going to be described as one through that first-person narrator's eyes, you can be darn sure.
This has caused for some editing on the fly, since I catch myself holding down the backspace button uttering "No, no, no. He wouldn't say that, especially about that guy." Or things to that effect.
One thing I really love about writing in the first person is that it allows me to dive into another character. It's up to me to transform and grow along with him, or he's not going to come off as realistic. Of course that means drawing on personal experiences and having that main guy do things I would've done (when I was his age, of course, since he's 17) in a few instances. The fun part is writing his reactions to other characters, and deciding what I want those reactions to be.
As usual, I'm learning as I go. Characters are great, and I think I've got a few good ones, but putting that much emphasis on one character in my first crack at a novel is a bit intimidating.