I loved the first chapter and even though the first didn't have the adults, the second is said to have the young versions in it as flashbacks.
Jaeden Lieberher and James McAvoy as Bill
They don't really look a like and I guess James will be doing an American accent. But basically Bill stays a cute lean nerd. James McAvoy is best known for Split and X-Men: First Class.
Sophia Lillis and Jessica Chastain as Beverly
Perfect casting in that her name came out in conversation in who to cast. Jesica Chastain is best known for Jurassic World.
Finn Wolfhard and Bill Hader as Richie
Perfect casting as Richie is a smartmouth snarker and they do kinda look a like. Bill Hader is best known for the Skeleton Twins, SNL, Cloudy with a chance of meatballs and Power Rangers.
Chosen Jacobs and Isaiah Mustafa as Mike
Mike is important in the book but didn't get much focus in the first movie. Isaiah Mustafa is known for Shadowhunters and those Old Spice commercials. He doesn't look much like Chosen but oh well.
Wyatt Oleff and Andy Bean as Stan
More or less look a like. Stan is the token Jew. If you already read the book or saw the original, you'd already know what happens to Stan. Andy Bean has been in Allegiant.
Jay Ryan and Jeremy Ray Taylor as Ben
Well since Ben is supposed to be chubby as a kid and hunk years later, it is obvious they don't have to look that much a like. Jay Ryan is New Zealand-Australian, he has been in Neighbours and CW Beauty and the Beast.
Jack Dylan Grazer and James Ranzone as Eddie
Perfect casting they do look a like. Ranzone was on The Wire.
Teach Grant and Nicholas Hamilton as Henry
Henry seemed to die in the first movie but they cast Nicholas Hamilton in the second so let's see what happens. They don't look a like but what can you do? Teach Grant is from Canada while Nicholas Hamilton is from Australia.