And Squall was all emo throughout the game and then he just suddenly changed, there was no progression and development imo, he just changed instantly at one point.
No offense, but you really missed the point on this one. For starters, Squall is not emo, and while we're on the subject being depressed about something does not automatically make you an emo, and emos aren't always depressing either, it's just a culture nothing more.
Squall is a very well developed character. He has lived a life full of despair and loss, he is not just acting that way to fit into a culture, he never had a family (that he can remember) he has spent his life in an orphanage, and from then on everyone ever mattered to him left in some way - he grew up surrounded by disappointment, and had no one around to support him, he wasn't emo, he was genuinely angry about all that had happened in his life. Because he knew how much it hurt to lose someone important to you, he eventually just gave up on the whole thing and decided that he wasn't going to get close to anyone anymore, if they just leave you (be it through death, or desertion, or whatever) then what's the point of being friends with anyone? However, he still has his emotions deep down, which is why we see his thoughts on screen.
Also he didn't "suddenly change at one point" - he stayed the exact same person throughout, he just became more open. When Rinoa comes on to the scene, Squall is in love, he may not realize it at first, but that is what's happening, and can be seen as early as disk 1. Throughout their scenes together, we can see from his thoughts that the more he gets to know her, the more he wants to open up to her, to tell her he loves her, but he can't, because he's afraid, simple as that, he's afraid that something will happen again that will pull them apart. The whole thing goes on while he is still the Squall that you wrongly describe as emo; he can't risk getting close to her, because he knows how painful it will be when he loses her.
If you look, during the FH scene, he is trying to open up more (again look to his thoughts for this) but the personality he has given himself is stopping him from doing so. You may also notice that he becomes slightly more open after the orphanage scene, it's like he feels more connected to the others now, seeing as they've had the same experience he has, and have all turned out alright, and furthermore, he hasn't lost anyone after all - they're all still here. The scene were Rinoa goes into a coma is also a revelation for Squall - he realizes that even when he refuses to allow Rinoa into his life, he still cares about her, and it will still hurt if she dies, and furthermore if she does it will be far worse than it would have been if he
had allowed her into his life, at least then he would have had something out of it, to remember when she's gone.
Finally there's the "Eyes On Me" scene where he finally opens up about everything - this is one of the most powerful moments in Final Fantasy, and if you listen to the lyrics they actually match Squall and Rinoa's relationship perfectly (even though in storyline the song was written by Julia, it was written by someone in our world, so they may have intended this) I didn't see it as sudden at all, I felt like the whole game was building up to it, it just made sense to me.
Sorry to go off topic, but I needed to say this.