The Harry Potter series was forced to tackle numerous monumental problems. The first, and perhaps most obvious one, was bringing the magical world to life. Spells, Hogwarts, magic cars, giant three-headed dogs...the list goes on and on.
But a more subtle problem was the actors. The production involved a very long audition process, as finding the rights actors was paramount to the movies' success. Unfortunately, child acting can prove quite temperamental. Some children are born naturals, whereas others need a bit more work and practice. Just take a look at these examples for instance...
Bonnie Wright
It may have been miscasting, it may be bad acting, and it may have been poor screenwriting, but Bonnie Wright simply did not fit the role of Ginny Weasley. Ginny doesn't have much to do until Order of the Phoenix, but even then she's relegated to a side character.
Her role expands greatly with The Half-Blood Prince, but she and Harry do not make for a compatible duo. Their chemistry is all wrong, and Wright is rather flat and emotionless in the role.
Daniel Radcliffe
With that said, Daniel Radcliffe isn't much better. Some people may find this to be a bit odd, considering he's the star of the franchise. Now, Radcliffe certainly came into his own in the later films. By Prisoner of Azkaban, he was quite commendable, and by the later movies, he was downright excellent.
However, his performances in The Philosopher's Stone and Chamber of Secrets are spotty at best - with the former being particularly bad. Some of his line readings are questionable, and he fails to convey the necessary emotion for many scenes - including the pivotal Mirror of Erised sequence.
Matthew Lewis
Matthew Lewis is most well known for his looks. It's no secret that "Neville got hot." And it's a shame because he can be quite a good actor. Lewis showed some natural comedic talent in the first couple of movies - especially the first, with some funny line readings like, "I can't remember what I've forgotten."
However, some of his scenes also fall a little flat. His lingering parental pain never really lands, and his epic speech in the final movie is less than convincing. He's certainly not bad, but there were far better child performers.
Emma Watson
Emma Watson suffered a reverse fate to Daniel Radcliffe. Whereas Radcliffe got progressively better throughout the movies, Watson became worse. She makes an exceptional Hermione in the first two or three movies - smart, knowledgeable, but also a bit too conceited and bossy.
By the later movies, Watson (and Hermione) wasn't nearly as convincing - or as enjoyable to watch. Perhaps this is due to the series' increasing divergence from the source material, or perhaps it's because Watson tends to over-act.
Harry Melling
Unfortunately, the movies didn't do a lot with Dudley Dursley. He plays a prominent role in the first movie but then all-but disappears until Order of the Phoenix. With that said, Harry Melling does a great job in the role. He is convincingly spoiled and bratty in the first movie and brute in the fifth.
He also shows a good range of emotion - when the clouds grow darker in Order, he nicely conveys a sense of fear and confusion. Add in some enjoyable slapstick (like his doe-eyed walk with Harry in Order), and one has an excellent performer.
Devon Murray
Devon Murray plays Seamus throughout the entire series, and the casting director couldn't have found a better actor. Granted, Seamus doesn't get much to do.
However, Murray displays a commendable and natural talent for comedy (and acting). He is quite a funny performer, as evident by his timing and enjoyable line delivery ("That's nothing to go on..."). But he can also convey frustration, anger, and reluctant acceptance, as seen throughout Order of the Phoenix.
Frank Dillane
For those who don't know, Frank Dillane played the 15-year-old Tom Riddle in The Half-Blood Prince flashback sequences. And he was simply magnetic. His scenes with Slughorn are exceptional.
Watch the way he slyly smiles, curves his lips, and shakes his head before he says "intuition." Or the way he widens his eyes, desperate for an answer when he asks Slughorn about the protection that Horcruxes offer. He's scary, he's smart, and he's sly - in short, he's the perfect Tom Riddle.
James & Oliver Phelps
Fred and George Weasley largely serve as comedic relief, so it was paramount to get actors who could effortlessly convey comedy. Luckily, James and Oliver Phelps were a jackpot. They share an undeniable and electric chemistry (probably has something to do with being twins...), and their screen presence is both natural and entertaining.
They embody the roles through good comedic timing and some wonderful line delivery. But they are also game for some "serious" acting, such as their "putting on a brave face" scene in Deathly Hallows where George asks Fred if he's OK.
Tom Felton
Draco Malfoy is a complicated character, and Tom Felton brilliantly portrays these complex emotions. In the earlier films, he perfectly conveys Draco's spoiled and slimy personality, and he makes for a convincing bully. In the later movies, he expertly conveys the moments of Draco's hesitation.
Watch the way he can barely hold himself together when facing Dumbledore, the look of shock when seeing Katie Bell in the Great Hall, or the look he gives Harry in Malfoy Manor. The filmmakers struck gold with Tom Felton and his consistency.
Rupert Grint
Rupert Grint is by far the most natural performer (at least out of the kids). He is excellent from minute one, and he is by far the most "natural" actor of the early movies. He's far better than Radcliffe, and he doesn't tend to over-act like Watson. He also grows into the most commendable adult performer of the three - a feat he proudly shows off in Deathly Hallows Part 1.
He convincingly conveys a wide range of emotions, and he fits into the role like a glove. Audiences see Ron, not Rupert Grint as Ron. The same can't be said for either Radcliffe or Watson.