Writing comedy can actually be a lot less fun than the finished product implies because you have to work hard to make sure every joke counts. However, the Deadpool movies are actually easier to write than most. This is because the writers aren't locked into the jokes that Ryan Reynolds actually uses on set. Because he's wearing a mask, all his dialogue has to be re-recorded anyway, meaning the writers can always add in new lines if they come up with better jokes during post-production. This is exactly what happened while making Deadpool 2. As writer Rhett Reese explains in Deadpool 2 Blu-ray special feature...
In post-production we rewrote a lot of Deadpool's dialogue. He's behind a mask so you can kind of place whatever words you want to into his mouth. We did the same for Colossus because he's a CG character.
When you watch all the behind-the-scenes material that's included in the new Blu-ray release of Deadpool 2 you see how much the movie was really always being workshopped. Every scene tries different jokes and every joke gets tweaked in different ways trying to find the best way to deliver it for maximum laughs. While most movies would be stuck with whatever they came up with on the set, Deadpool 2 had some additional breathing room, as they could actually change the lines Ryan Reynolds said since they didn't have to actually match lip movements.
However, while it is clear that new jokes and alternative versions of the existing ones were continually tried on the set, Ryan Reynolds makes it clear that what was happening on the set wasn't technically improv. In one of the Deadpool 2 Blu-Ray special features Reynolds confesses he wasn't just throwing out new jokes on the fly, he wrote down all his ideas as they happened...
I think there's a misconception that I am some kind of improv guy. I love improv I, I did improv for years, it was how I started, but what I do when come on set and I have, say 10 or 12 alts to one joke, is that they're all written down before. So it's not like I'm just thinking of them, firing them off one by one.
Whether the jokes come off the cuff or one at a time over a period of time, the key is that they come at all. Of course, the fact that they had the benefit of being able to be added to the movie incredibly late in production means that every funny moment has the potential to have the funniest possible line. Perhaps that's why both Deadpool movies have been so well regarded for their humor, they never stop trying to improve it, even going so far as to release an additional version of the film to try and be even funnier.
