When it comes to 99% of movies, genre is pretty black and white. Based on the story and the setting, there's a certain number of tropes and archetypes that are expected to feature, and even the most innovative examples tend to stick pretty close to at least a few conventions.
The more genres you try and add into the melting pot, the more difficult it becomes to balance them successfully , and in plenty of cases trying to hard to bring something new to the table can backfire and leave the whole thing as a mess. However, there have been more than a few titles over the years where it appears that the creative team simply decided to down tools halfway through, tear up the script and set the rest of the narrative in another genre entirely.
Admittedly, it doesn't always yield spectacular results, but when it works it gives the movie in question a completely new lease of life, throwing the audience for six as things take a drastic left turn into completely uncharted territory that nobody was expecting during the opening credits.
On paper, Hancock had the potential to be a phenomenal deconstruction of the superhero genre, with the focus placed on an all-powerful being who by all intents and purposes was a total pr*ck.
Unfortunately, this was during the time when star Will Smith was steadfast in his refusal to deviate from his established screen persona during the summer months, so despite a hugely promising first act the movie soon devolves into the standard tropes we've seen a thousand times before.
Hancock makes the switch from exploring what forces a superhero into becoming an alcoholic layabout with no interest in saving the day to a formulaic tale of two characters with the same powers using them in a third-act showdown heavy on visual effects. The tonal change was clearly a studio-mandated one, and it sinks the whole movie as a result.