I watched the movie from the get-go, so I have no idea whether or not there are differences compared to the series. These are just my thoughts about the story. I haven't read the webtoon either.

The premises of the story are interesting, and the very beginning was funny. I was expecting it to be a romantic comedy for the most part: even though some parts were more embarrassing than fun, I didn't mind all that much. The characters all had the potential to make the story lively and memorable as well.
...so I really don't understand why did the writers decide to set all the elements of the plot and the interesting quirks of the characters aside, only to utilize two of the most annoying plot devices in existence (the miscommunication and misunderstanding tropes) and create one of the most boring BLs I've watched.
Out of all the elements in the synopsis (Seon Ho being an adult webtoon artist, him having lost his libido entirely until Mu Young becomes his assistant, Mu Young secretly being his fan), NOTHING is relevant in the plot. At best they are background elements, only introduced and not brought up for almost the entire film.
These are all things that I could easily let go of if the story was still good in some way, but it just isn't. Nothing relevant happens in most of the film. Unfortunately even the actors didn't do much to make it better: they spend more time thinking about what they (allegedly) feel, rather than showing it to us at any moment. Even when things actually happen in the story, the acting and the poor editing made the events feel unimportant and bland.

I don't recommend watching this nor the series. I'm usually nicer with shows that I was able to complete and give them at least a 6 out of 10, but this time around I can't. At some point I was feeling so bored and annoyed with everything that I almost dropped this title entirely, so I can't even say that I was entertained.