X marks the spot. - When Marie shot the arrow, and it was redirected to the mirror in the open book; devised to bring her to Replicate Castle. Wasn't explained until later by Hussain and "Nutik".
Never trust a stranger. - Used by Marie when she first met Haven, and when she later allied with the Black Knight.
Out of sight, out of mind. - This was the structure used to keep the Idiom Dalis, in control. Her point was to help maintain control over other Idioms, but to prevent her from using her powers on those she wasn't dictated to, she was presented with candles. The light made her close her eyes, for light gives way to darkness, and lets the mind clear and see better.
Knock on wood. - This was when Haven tapped his handle to the floorboards, trying to arouse the hidden Idiom. Considering that it didn't work, it was not attempted again.
Cold shoulder. - When Haven and Marie were sitting on the steps, having come through an Idiom inhabited room earlier, and Haven proceeded to ignore her. Marie noted the little wet spot on his shoulder.
Give up the ghost. - This was more a representation of an Idiom, and gave way to the foundation of the "Ghoul", that Haven faced off against when it took hold of the painting.
It's better to light a candle than to curse the darkness. - Though not indicated in this story, this was the phrase that the Idioms kept rumbling through Marie's head, and in her fear, she tried to light the painting with the Ghoul on fire, until Haven forced her back.
Thou shalt not kill. - (sort of like "Thou shall now wear a kilt". lol, Robin Williams reference). This is a Saying, a law that helps to keep the structure within Replicate Castle. Fact of the matter is, without it, the Watchman would have died long beforehand.
Just in time. - This Idiom, Alexiyana, was made to help keep the balance, and prevent death through the healing process. She's not made to talk, and heals through the use of her tears, which freeze upon physical contact. Alexiyana inserted a frozen tear into Haven's shoulder wound, and as ice overtook the shoulder, she cracked it to reveal nothing.
I have not slept one wink. - This was just a joke that Haven made that Marie didn't pick up upon, considering he had just been saved by Alexiyana. It holds no relevence or use whatsoever.
Many are called but few are chosen. - This was indicated by Haven, when Marie was questioning him about his post. Though not further inclined by much, there had been a group of Chosens who could have been Watchman, but in the end, it came down to just one.
Let the cat out of the bag. - This Idiom was made to be a useful tool to the Watchman, to find and discover something. However, to prevent taking advantage of "Naic", something would have to be returned. Despite that he is an Idiom, Naic proves to be a useful and good friend to both Marie and Haven. His glowing eyes are another usage of his saying, where he reveals what remains hidden in the darkness. As for swimming...he's just made to be a good swimmer.
Camera never lies. - This is where the Idiom "Rolls", was thrown down from the ceiling. Designed to keep an eye on what lay within the castle, Rolls was constructed to answer to any questions that it could truthfully. However, sensing the fear and darkness sweeping around the Book of Ages, and the Idiom within it, he was afraid to answer Haven's questions, in fear of disrupting the balance.
The customer is always right. - Upon entering a small, familiar world, the Watchman discovers that the Idioms are being strewn throughout the worlds slowly. Examples of the obstruction have already been shown; the Book of Ages copy, X marks the spot (later indicated), the Ghoul, the Black Knight and his leaders, and of course, now the Customer is always right. This Idiom is indicated as pretty much harmless, but gives warning that there are great matters of atrocity happening here. "Kez" later helps Marie to recover Haven's true self.
A picture paints a thousand words. - In this case two thousand. The Idiom, "Milena", was designed to store all the information of previous Watchman. It is the construct of their souls and minds, the soul representation of the history of Replicate Castle and the Idioms. It helps to maintain the balance, and guards the path between Replicate Castle, and Baron Fortress. Knowing that the Watchman's time was drawing to an end, Milena was directed to allow Haven through and to his waiting duties.
Keep your allies close, and your enemies closer. - This was used when Barqs, the Black Knight, and Marie, Eleanor, and Naic teamed up. Though the girls didn't have this in mind, it was presented by Barqs' offer of alliance.
A place for everything and everything in its place. - This wasn't indicated that much, but Marie notes when being escorted by the Baron guards, that everything seemed to fit in perfectly within the castle.
Keep your chin up. - This was an Idiom that Naic had ran to retrieve. Represented as an unnamed, little boy, he used Marie's rising hope to release her before disappearing and returning to his post.
Music soothes the savage beast. - On instinct, Vahn had lent the flute over to Marie, and with it, released from her chains up on Baron Fortress, played it to help calm Haven and return him from the dark depths of his miseries.
Love conquers all. - Having realized their feelings for one another, Haven and Marie were able to pass through Baron Fortress without trouble, and return to Vahn's world.
My brother's keeper. - This wasn't stated, but when Haven was reliving his memories as a child, Hussain remained deemed the unworthy of the twins. Haven being the Chosen, he had to do little for himself, leaving Hussain to the troubles and tasks.
Live by the sword, die by the sword/Fall on your sword. - These are indicated as laws in Replicate Castle. When the Baron guards tell Hussain that Eleanor escaped, he pushed them over the boundary into the castle, which caused their deaths.
Mad as a hatter. - This was one of the most powerful Idioms, made to be sealed away in the tower opposite of the original Book of Ages. However, needless to say, through the work of Hussain and Nutik, the Mad Hare managed to break through as Haven's power was flaunted and his duties stripped from him. The Mad Hare took hold of the Book of Enlightment, and resided within the sealed room of the copies of Books of Ages, and proceeded to tear away at the history, boundaries, and laws of Replicate Castle.
Saved by the bell. - Each ring of the Bell indicated when the Mad Hare ripped away the pages of the Book of Enlightment. Those who had previously stepped foot into Baron Fortress could hear it calling them. It drove the Idioms to chaos, and it wasn't until the Bell cracked and broke, that they were "saved" from the Hare's madness. The Bell was a representation of things gone wrong.
Bell, book, and candle. - Okay, this was the title and foundation of this story. The Bell was the indication of chaos and torn away laws and history. The Book was the Book of Enlightment, that held those laws and pieces of history. The Candle was the only thing capable of destruction to the Idioms.