Sum together all the gore present in this film and it still won't get any close to the excruciating painful experience of watching the film until the end.
Most of the character's motivations are either non-existent or nonsensical, you're just unable to relate or care to any of them, you just want it to end.
The most charismatic character is Malcom (Michael Sheen), but he got so little screen time that ended up remaining just a mildly interesting promise and misused by the writers - which is still the most interesting any character could get.
On the other hand, the protagonist Thomas (Dan Stevens) was just as bland and generic as you could possibly get, and had massive disproportionate amounts of time onscreen, making the 2 hours even harder to finish.
It felt like you were invited to a huge feast where they promised to serve you delicious juicy, bloody steaks, but instead they force-fed you with never-ending servings of unsalted cold mashed potatoes for hours that seemed like a life-time.
Lucy Boynton's character Andrea were specially poorly written. Seems like you're watching a movie back from a time where no one expected female characters being portrayed with more than a single dimension and the writers didn't cared about it anyway.
The only characters that were able to make you feel something closely resembling empathy were Ffion (Kristine Froseth) and Jeremy (Bill Milner). But again, too little time onscreen.
It's just sad because they had such well-crafted visual development, specially the setting design, the costumes design and the general visual effects. They made the overall experience a little less unbearable, but deserved better.