Dishonored is a very very important academic study for every professional and wannabe game developer who cares about game writing - which, in a perfect world, would be fucking all of them. Dishonored illustrates the vital role that dialogue plays in videogame story building. It does that by doing absolutely everything right in every aspect of storytelling except the dialogue, and with that single deficiency all that effort goes completely to waste.
The world building on display in Dishonored is actually really bloody impressive. You can tell that they sat down and worked out every aspect of this fictional universe. You can find maps of the entire world despite never actually leaving the one island nation (incidentally: fucking cocktease, Arkane), there's a clear political and class system, current events like the plague are permeating every aspect of your surroundings, and everywhere you look there are written documents and detailed posters adding more and more background.
But even if you absolutely kill yourself putting those details together, the most it can do is put your in-game world on the same level as a ghost town. All that is merely evidence of stuff having happened in the past. The next step is to make your world feel like things are still happening in the present, that it's a living breathing society full of individuals, and that's where dialogue and character interaction comes in. The characters in Dishonored have faces and costumes that show a great deal of thought and care going into visual design, but when it's time for dialogue they all turn into robots.
Part of it's the performances. This is the risk you take when you hire big name screen actors for prestige points rather than career voice actors, because voice acting and film acting are subtly different disciplines. Yes, people like Nolan North and Robin Atkin Downes might be a bit overexposed in videogame voice circles but that's because they know how to inject a bit of life into a performance. There's a bit in Dishonored where you catch a bloke peeping on someone in the bath, but going from his line delivery you'd think he'd been caught underlining something one too many times in the accounts ledger. Then there's visual performance, of course. Conversations between the player character and his allies tend to be held Oblivion-style with the characters maintaining fixed creepy eye contact.
But the dialogue writer doesn't get let off, because even if you did get in the most professional jobbing voice actor on the books, the lines in Dishonored wouldn't give them much to work with. Most of them read like the writer set out with precisely one goal in mind - to establish the next instruction or plot point or explanation of current events - and then they put pen to paper and stampeded straight for it like a fucking rhino on a sled. No asides, no ums or ers, no small talk, nothing that characterises actual human conversation.
The problem with a lot of game writing these days is that dialogue is used solely for exposition. Particularly in RPGs with dialog menus that consist only of going down the list raising topic after topic and listening to someone blithely read out encyclopaedia entries. Play a Lucasarts adventure game for a crash course in how to do dialogue trees. Half, perhaps even most of the lines don't have the slightest expositional purpose, such as when exchanging woodchuck-related tongue twisters with the carpenter in Monkey Island 2 until one of you tells the other to shut up, but it accounts for a massive percentage of the appeal, humor and unique personality of the games.