After spending the better half of a decade trying to get Deadpool on the big screen (in his proper form), Ryan Reyonlds had a strong intent on getting the film as perfect as possible. Not only has Ryan Reynolds strongly suggested that he (or someone very close to the project) leaked the test footage that basically got the film greenlit, but he even went out of his way and paid the writers to come on set when Fox wouldn't.
Screenwriters Rhett Reese & Paul Wernick revealed this information when being interviewed on AMC's Geeking Out. "We were on set every day. Interestingly, Ryan wanted us there, we were on the project for six years. It was really a core creative team of us, Ryan, and the director Tim Miller. Fox, interestingly, wouldn’t pay for us to be on set. Ryan Reynolds paid out of his own money, out of his own pocket."
Deadpool had a pretty small budget for a superhero film (reportedly just $60 million), so it's not a big surprise that Fox didn't set aside money to bring the writers on set everyday. It's great to see Ryan Reynolds care so much about not only the character but the creative process behind the film and acknowledge that everyone is vital to the project!
Deadpool 2 is currently in the works and is set to release January 12th, 2018.