I feel like people are only half explaining this to you. So, in the 2009 movie x-men origins: wolverine, Ryan Reynolds played wade wilson, aka deadpool, aka the merc with a mouth. That last bit is the important part, Deadpool is notorious for never shutting up. While fighting, he's always making smart ass remarks, annoying his opponent with snarky or random jokes. What happened in the movie is this; Ryan Reynolds could only film the first scene he was in, after that he left for another project. They decided to then get a lookalike to film the final confrontation, where a hairless, dead eyed wade wilson makes his debut as a remote controlled zombie with arm swords, eye lasers and, because they didn't have Ryan Reynolds, a sewn mouth. This created a large amount of backlash against the film by fans of deadpool because how the fuck can you portray the "merc with a mouth" with a sewn mouth? Anyways, the film went on to bomb, it killed people's hopes for a deadpool film and 6 years later, here we are eagerly anticipating what looks to be a true-to-the-comic movie adaption of deadpool.