In with a new interview with Harry Potter producer David Heyman, Heyman briefly discusses the notable British thespians in the films and the fact that no matter the character or role they are all treated the same:
The smallest part is treated with the same attention and care as the lead roles. Not just in the casting, but also the directing.
Thanks to TLC for letting us know about another interview in which director David Yates says that he has found the person to play a young Riddle. The piece does mention that Ralph Fiennes' nephew, Titan Tiffin, has been cast as Tom Riddle:
I've found a really interesting kid to play Riddle. I can't tell you who it is yet - it's early days.
Please treat Yates' comments as rumor till confirmed by the studios.