Well, this ought to be amusing for me to give my own response to.
First, among the characters I had a slightly more active hand in...
I have read Le Fantôme de l’Opéra, by Gaston Leroux. That was for French IV, my senior year of high school.
I have taken in multiple versions of Don Juan; I'm most familiar with the Molière version (in French, he's actually known as Dom Juan, with the J pronounced; I believe there's a recording of me doing Poirot where you can hear this), though I did watch a performance of Don Giovanni as well as a film noir version of Dom Juan to see multiple takes on the same character. I think my push towards one particular take on DJ was probably the most strident I got with editing a character.
Interestingly enough, I did not read any Poirot work, nor did I read Alice In Wonderland, for the other characters that I did direct scripting. I did read some of the public domain releases for a quick scan on what kind of voice I should have, but not enough to have an authoritative voice on them. Though to my credit, I did watch a BBC production of Murder On The Orient Express, and I've seen a couple animated versions of Alice In Wonderland, so I didn't go in blind.
For characters where I didn't have as much of a direct hand...
I read Don Quixote in my sophomore year of college, as a part of my Classic European Literature I (LL250) course, which was a requirement for all foreign language majors back when I was in college. Great fun; I was so glad Quixote was one of our characters.
I have read quite a few Sherlock Holmes stories - A Scandal In Bohemia and The Five Orange Pips are personal favorites, though The Red-Haired League rings a bit close to home (hard to tell unless the light is just right, but I have chestnut hair). Interestingly, I haven't read either of the stories featuring Moriarty.
I've read bits of Frankenstein here and there, most recently as prep work for doing this project, but I've never been able to push through it all. I kind of like our version a bunch; I like to think of us as somehow making him happier even as things kept going wrong for him.
Peter Pan, I read when I was 11 or 12. It's been a while, but I do remember the character quite well. It helps that, for all that he's the title character, he isn't really the main character.
Romeo and Juliet... I read this twice in high school (once at my second high school, once at my third). Not to mention seeing multiple versions of it (including some truly horrible film versions). Not the Shakespeare play that I know the best (that'd be Macbeth; read it three times, once in each high school, plus several other readings in college to compare with Ubu Roi), but I certainly have read it enough to know it well and have strong feelings about it (in short, for all that I liked making people feel like crap for what happened when Romeo went to the hospital... he SO deserved it).
I've read a bunch of different takes on Arthurian legend, so I'm familiar with multiple versions of Morgan Le Fay - victim of usurpation, innocent slandered by relation, wicked temptress, mother of the downfall of Tintagel... so many versions to work with.
I've seen multiple adaptations of Gulliver's Travels, dozens of stories inspired by the tale (including Laputa), and I've read several of Swift's other works (including the infamous On Human Ordure), but I haven't read the original. It's on my list.
I've read The Wizard of Oz years ago; been ages since I have. I also have seen both the original film and the 80's Nightmare Fuel-filled Return To Oz; if we ever wanted things to get serious with Westie, I was going to cull from that for ideas (while listening to They Might Be Giants'
Hall Of Heads, of course).
I'll be honest; I'm most familiar with Treasure Island via Muppet Treasure Island. In my defense, Tim Curry.
I'm actually more familiar with the original story that inspired Sweeney Todd than the murderous barber himself; I just know that he has a more sympathetic backstory than the original (who moved quite a few penny dreadfuls in his day, if I may make the joke).
Finally, for the two background characters mentioned... I certainly read Cinderella, though not in many years. Probably will in a few more, after my young'un is both born and ready to hear fairy tales. I read Dracula back in 7th grade, and I think it's somewhat of a marvel of durability, given how badly it was written. But hey, some ideas are so good, they transcend poor execution.