In the latest "EMPIRE" magazine he goes into detail about this.
So I guess he was a big fan of DEADPOOL comics since filming blade. When he was given the choice and after thoroughly going through the script for wolverine film, he knew it was a shit take on DEADPOOL, and I guess it was around the time when the writers were on strike so that accounts for the script being so shit. But he also knew that they were in talks of making a Deadpool film after wolverines film.
He ended up pretty much saying fuck it, he wanted to play Deadpool so bad that a shitty version is better than no version at all. He also said filming was such of a shit show that they had him write all of his own dialogue( SO that was why he was actually very much on point while playing Wade Wilson, because he got to put HIS take on him, but when they changed him into "deadpool" is when it all went to crap). And they ended up with a bunch of stupid changes to Deadpool because the suits at Fox wanted him changed like that, they had like 10 versions of him and he kept getting more and more bastardized.
But I have faith in this one because they pretty much gave them free reign after the response to the leaked test footage. Ryan Reynolds may not be the greatest actor but he is perfect for this role. And he understand the character and is a fan of him. He has his heart in this one so I think this may actually be one of the truest comic to film adaptations we have ever see.