Spoiler warning!
This article contains plot spoilers. If you do not yet know of this information, it is recommended that you read at your own risk or not at all.
Shooting or hitting the bell on the Dreadful Wale's bridge will make it fall off, showing the disrepair state of the ship (this doesn't happen in Dishonored 2). After it fell, you can also kick it around like a ball, and it rings quite loudly too.
Billie's wanted posters change over the course of the game and do have variants based on your actions.
"Low Chaos" variant: This starts to appear from the 3rd mission if you were detected in the first two and if you didn't kill many people.
"High Chaos" variant: replaces the "low chaos" variant if you killed a lot of people.
"Ghost" variant: This appears in at least the first two missions, showing her normal right eye. If you don't let any enemy see you, however, this will replace the previous two variants in later missions since nobody saw Billie with the artificial eye.
"Hollow" variant: This only appears as a hollow in the Bank Job mission, showing Billie in an alternate timeline, still missing an eye and an arm, and is also wanted for the murder of Jessamine Kaldwin.
Many of the female Eyeless members were former Brigmore Witches and you can see scars on their skins where the flowers and vines used to be.
In The Knife of Dunwall, during the prologue of A Captain of Industry mission, Daud mentions that "Lurk was especially good at uncovering [secrets]". Arkane probably built upon this quote to develop Billie's ability to talk to the rats and find out information no one else can.
The rats will scream when you harm them while listening to them.
If you pay attention to the rats, there are tons of interesting titbits to be gleaned. A list of their quotes can be found here. The most interesting ones, in my opinion, are:
Rats and Bloodlies are natural enemies: "Follow bloodflies, watch for smell of nests, papery and full of grubs. Spear us with their mouths if they catch us there! But delicious grubs..."
Rats told their young of the Rat Plague: "Old ones spoke of old days. Bad days. Sick. Madness in our mouths. Death in our minds. Armies of us in streets. In sewers. All feared us. Bite! Bite! Bite! Bite!"
And even remember the Lonely Rat Boy: "Old ones told of him. Little man, chittering like us. One of us. First one. Only one."
Clockwork Soldiers: "Metal man, knife hands. Steel feet stomp metal floor. Careful! It starts hunts. Never stops."
Electrified Floor: "Hush. Listen! Humming from the floor. Danger? Wary steps on wires. Shock! Twists us, fires in our minds. Burns!"
Ivan Jacobi's poster, protesting against regraving.
They eat dug up corpses, referring to the practice of regraving in Karnaca: "Bodies hidden in dirt, under marked white stone, rotting alone. Bodies unearthed, flesh in clean sunshine. Left for us. Sweet taste of decay."
Sanguine transfusion, a ritual practiced by the Eyeless: "Warm blood, easy blood. Draining from flesh. Drip, drip, drip into tubes. Flowing, him to her. Wait, wait. Drag the empty dead away. We swarm. Sweet blood warms our bellies."
They might say "Garbage, garbage, garbage.", which is something Granny Rag usually said, suggesting that they still remember her.
(In The Stolen Archive) Overseers: "Men with metal faces send hounds, yes. Watch for wet smell, watch for smell of fat and blood and burning."
(In A Hole in the World) Referring to the shifting nature of Shindaerey North Quary: "Burrowed here, but not always. Some days stone, some days iron. Shiver, then still. Air tastes like blood, then nothing but dust."
(In A Hole in the World) Referring to the Eye of the Dead God: "Run and hide, find the small places. Still the huge eye sees! The huge eye sees. Drags us to the dark places, drags us to the cold. Everything stone, everything old."
(In A Hole in the World) The Envisioned: "Soft flesh for biting, then cold stone, hard! Tastes bad. Breaks teeth. Have no blood in them anymore. Now they are always."
Sébastien Mitton mentioned in a tweet that this clockwork soldier doodle was drawn by David DiGiacomo's son. David DiGiacomo is the level architect of the Clockwork Mansion. This doodle can also be found in Deathloop.
The Voltaic Gun uses 5 different types of ammo (crossbow bolts, bullets, nails, scalpels, and fountain pens) and they are actually shown on the weapons. These types of ammo also behave differently:
Bullets and nails always break on impact.
Crossbow bolts, scalpels, and fountain pens can sometimes be retrieved.
They all break with charged shot, however.
Attaching a hook mine to a bottle will turn it into a homing bottle.
If a Hook Mine is placed in front of you and someone throw a grenade at you, the hook mine will actually grab the grenade.
Two non-lethal hook mines can be placed in close proximity to pull apart a person, killing them. [Credit: StealthGamerBR]
Hyperbaric grenades when thrown into a room, can push any button within the detonation range. This is particularly useful when a line of sight to the button is not available or there is no ammo left for the Voltaic Gun.
Hyperbaric grandes will force the clockwork sentinels to dig their blade-arms into the ground in order to stay in place, similar to how clockwork soldiers react to Windblast. The second hyperbaric grande, however, will destroy them completely.
When using Semblance, depending on who you are impersonating, you can trigger unique dialogues with certain groups of people. For example, aristocrats will talk to you if you're also impersonating an aristocrat but they will tell you to go away if you're posing as a lower class citizen; guards might salute you if you're an officer; or Oracular Sisters will say things like "You are forbidden from our sight" if you're pretending to be an Overseer. A list of such dialogues can be found here.
When you masquarade as a grand guard and stand close to another one, one of the phrases they would say is "We should gather for whisky and cigars tonight." This is a direct nod to a similar phrase: "Should we gather for whiskey and cigars tonight?" which was uttered so frequently by Dunwall's City Watch Officers that it became a meme.
You can again access the following locations by using Semblance to impersonate these individuals:
Spector Club: Eolina Rey or an Eyeless
Shan Yun's House: you still need the secret knock to enter the front door but if you're impersonating Eolina Rey, the maid won't be alarmed.
Security area of the Dolores Michaels Deposit & Loan Bank: use Semblance on the female offer on the second floor then talk to chief Alexander Peña in the basement to gain the access code
Door to the basement of the Royal Conservatory: An Overseer (I haven't tested yet but I bet Oracular sisters work too)
Much like Daud's immunity to Corvo's Stop Time, he's also immune to Billie's Foresight's time-freezing effect. While using Foresight near Daud, he will look directly at Billie's spirit, and follow her while she moves around.
I believe that Outsider isn't indifferent about what's happening in the physical world. He doesn't just give his Marks for fun but he has been actively trying to plot his freedom, and if that's not possible, his own demise, in order to free himself from an eternity of imprisonment. There are a number of evidence that can support this theory:
The reason why he gave people his marks is not because he found them interesting. He could see the future, or rather he saw the possibilities and outcomes of each person's actions. This is evidenced by his words from one of the Hollows: "What will it be like, to finally experience an ending? I see forever, and even I can't see that future." Because of this ability, he chose those who would change history in his favor. For example, according to Harvey Smith, the Lonely Rat Boy was marked because he is "pivotal (or potentially pivotal) to history" and was "in a position to affect many people or major events." So all the people whom he marked, through their own actions, have one step at a time helped him reach his final goal.
In The Knife of Dunwall, Eminent Domain mission, he also said this when Daud visits his shrine: "I see everything. I see forever."
He said the following when Billie interact with another Hollow "A thousand lives, a thousand choices, all converged on one wounded body, one wounded mind.""A thousand lives, a thousand choices"" refer to the people he marked and their actions. "This one wounded body, one wounded mind" is no other than Billie, the final piece of the puzzle that he needed.
The Outsider himself gave her all the crucial tools to help her reach and/or kill him. Without the powers, Billie would have a really hard time getting the Knife (the only thing that can kill him), and finding the location of Shindaerey North Quary. Without the Sliver of the Eye, Billie would never reach the Eye of the Dead God, and gained the ability to locate the Ritual Hold.
