It's a tricky situation -- Harry Potter and the Half-Blood Prince is a strong entry in the series about the boy wizard, but won't work as a stand-alone film. No background is provided, so audience members must have seen the previous movies or read the J.K. Rowling books or they'll be lost. Fortunately, the movie doesn't assume you've read the sixth book -- there are no gaps in the story that require the novel to fill.
In case you haven't read the book, Harry Potter (Daniel Radcliffe) returns to Hogwarts in Half-Blood Prince for his sixth year of study, while Death Eaters terrorize both the magical community and the ordinary Muggle world. Dumbledore (Michael Gambon) trusts Harry with increasingly important tasks in the fight against Voldemort ... and Harry is also learning a lot from an old Potions textbook that contains clever notes from someone self-styled the "Half-Blood Prince." Harry's best friend Ron (Rupert Grint) is taking lessons in Quidditch, self-confidence, and romance -- and why is Hermione (Emma Watson) displeased? Draco lurks in hallways and towers looking pale and conflicted. That description makes it sound like I'm recounting a teenage soap opera. To be honest, there's a lot of teen romance in a movie that's also about battling the dark forces. The movie takes a more lighthearted attitude than the book in this arena, although Lavender -- who has a crush on Ron -- is cartoonishly oversensitive. Luna, on the other hand, gets some nice moments without resorting to broad humor. The quieter, smaller relationship scenes generally work better than the comic ones, and I especially liked a scene with Harry and Hermione on the stairs, escaping from a trying party.
Screenwriter Steve Kloves, who has adapted most of the Harry Potter books at this point, has neatly pared down and reshaped the novel to fit in a feature film structure and length. Instead of trying to squeeze all the Hogwarts goodness into 2.5 hours, he has cut whole subplots and minor characters so we can focus on the most important elements of the tale. As a fan of the book, I missed Bill and Fleur, wished Neville had something to do, and was sorry not to see Emma Thompson as Professor Trelawney. The house-elves are also gone, and the politics of the Ministry for Magic -- with some of these omissions, it'll take mighty smart scripting to hit certain plot points in the adaptation of the final book.
Kloves and director David Yates also have included a number of short scenes and visual hints to prepare us for certain characters or to set the mood. Harry's early scene in a cafe strikes me as a better key to his current feelings and state of mind than the sequence from the book with the Dursleys. The novel's recurring scenes where the characters read about Death Eater experiences in the paper is replaced by an experience of their own. A quick shot of a "Wanted" poster for Fenrir Greyback provides an introduction to that character for when he appears later.
On the other hand, the film wastes time with Bellatrix Lestrange in Helena Bonham-Carter's excessively over-the-top performance. Other actors can get away with overdramatizing their character somewhat -- Alan Rickman as Snape is addicted to meaningful pauses mid-sentence -- but the characters' warmth or charm wins in the end. Bellatrix is too chaotically crazy to endure for long. Other downsides to the movie include a few wincingly predictable moments -- one climactic scene involves a "shock" moment right out of a bad horror movie, and there's a plummet that looks like it belongs in the Lord of the RIngs films.
Jim Broadbent joins the cast in this film as Professor Slughorn -- it's always delightful to see yet another veteran British character actor appearing in the Harry Potter series. Broadbent's Slughorn is a little less insinuating and power-hungry than the character in the book, and less rotund than I'd imagined the character, but I have no objection to these improvements.
Fans of the book may feel unhappy that some of their favorite parts are changed or omitted. However, as a movie dependent on earlier films in the series, Half-Blood Prince provides a carefully plotted story with plenty of humor to counterbalance the darker moments. Yates, who also directed Harry Potter and the Order of the Phoenix, has managed to find the best way to tell the Harry Potter stories effectively on film -- as a tight suspense film that isn't too scary for older kids, with a good dose of humor, Young Love, and charm. I'm not going to compare and rank this film up against the others in the series (better than #4, #3 still best) because I haven't seen some of them in years, but this movie has me looking forward to the final two in the series, and even hoping that some of the seventh book's flaws will vanish in the movie adaptations.