I haven’t thought about all the characters in all these books, actually. But the characters I have thought about from The Mortal Instruments…
Clary Fray/insert-millions-of-surnames-here - I actually seriously always imagined her as Lily Collins, because of this picture I saw of her a long time ago with crazy red hair and curls. But I also think Molly C Quinn- she’s from Castle- would be great for the role as well. Not the most popular opinions.
Simon Lewis - I just made a post about this. If he was still as dorky as he used to be, Kristopher Turner. Or a slightly older version of Callan McAuliffe.
Jace Herondale - I really loved the idea of Alex Pettyfer for him, seeing as that’s who I imagined playing Jace as I read the books.. But now, I’m kind of in love with the idea of Jamie Campbell-Bower playing him. I really actually wanted Jake Abel to play Jace at one point, because he can play those kind of people really well- of course, that’s probably the most unpopular opinion in existence, but it’s true.
Isabelle Lightwood - If she was an actress, I would love Anna Christine Speckhart to play Isabelle. Kaya Scodelario would be good as well- and if she acted, maybe this one model VeroniCA LAvery. NOT MEGAN FOX, that’s for sure. (I love her. But. No.)
Alec Lightwood - Nicholas Hoult. End story.
Magnus Bane - There’s this one Asian guy, Myavi or something like that, who I always imagined as Magnus.
Jocelyn Fray/Garroway/insert-millions-of-surnames-here - Bryce Dallas Howard? She’s pretty, and she looks old enough to be a mom in some pictures while keeping that beauty. I never gave Jocelyn much thought. Maybe Julianne Moore.
Valentine - Can we just have Daniel Craig swoop in and take this role in all his badass glory? Thanks.
Maryse Lightwood - I seriously always thought of a mean Lily Evangeline when I was reading anything that had to do with Maryse. I never gave much thought about her husband, though.
Sebastian/Johnathan - I have no clue. Sometimes I think Ben Barnes, sometimes I think a black haired version of whoever the hell is going to play Jace. c: (So, in this case, a black haired Jamie.)
Maia Roberts - I really like the model Lily Aldridge, but sometimes, I feel like it’s a big no. Maybe someone like Naya Rivera. I’m never sure.
Jordan Kyle whatever - (I never get his name right. .-.) Joey Richter! Please?!
Camille - Candice Swanepoel, Caroline Trentini, Doutzen Kroes or Lily Donaldson. I know, I know. They’re all models. But if they acted… (I think Doutzen acts. I’m not sure.)

AS FOR THE INFERNAL DEVICES, I’ve only had time to read the first book. and I never thought about a dream cast for them. but so far:
Tessa Gray - GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON GEMMA ARTERTON.
William Herondale - AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON AARON JOHNSON
Jem Carstairs (or is it Castairs? I always spell it wrong.) - :c I have no clue, yo.
Jessamine Lovelace - Might I just say, I love her name. Anyway.. Tasmin Egerton. Or Amber Heard. Or Teresa Palmer. So many blonde women, so many choices.
Camille - See The Mortal Instruments section of this answer.
Magnus - See The Mortal Instruments sction of this answer.