Who's Poe? We Just Love Oscar.
There's Rey, there's Finn…and then there's this less-than-pleasant fellow. Though part of the new trio replacing Luke, Han, and Leia, Poe has less meat on his bones, character-wise, than the other two. He's basically there to send Finn on his way at the beginning and blow up Starkiller Base at the end.
That makes him as much of a plot device as a character, and it takes some doing to make him someone worth relating to.
And that's part of where getting a good actor can really make a difference. Oscar Isaac can bring a whole lot of personality to a character who's really kind of flat on the page. This good guy helps Finn escape, delivers some key info about BB-8, and fare thee well.
But Isaac finds little quirks and cleverness that lets us feel this guy's bravado, his sense of derring-do, and the way that he—as an X-wing pilot—will happily lay it all on the line for the sake of the Resistance.
A great example comes early on, when he's been captured by Kylo Ren and is facing grievous Force-poking to get him to talk:
POE: Who talks first? Do you talk first?
KYLO REN: The old man gave it to you.
POE: It's just very hard to understand you with all the...
The lines are full of snark, but it's up to the actor to give them personality. You can see how fearless he is, how he'd rather make a joke and seriously get this guy mad than quiver and tremble in the face of it.
That means that he actually becomes the third leg of the stool along with Finn and Rey, the new trio to replace Luke, Han, and Leia as the centerpiece of the story. Poe may not have much going on in this film, but we suspect we're going to see a lot more of him going forward.