I’ve been looking over some of the choices suggested here.
Helen McCrory was only suggested by The Mirror, not really a reliable source. The Mirror is the kind of paper which inspired the character of Rita Skeeter.
I remember before Goblet of Fire came out, people had Rowan Atkinson as Voldemort, though the other rumour – John Malkovich – was more intriguing.
Still, McCrory definitely looks the part. If she can act, too then she has my vote. I’ve only seen her bit part in Interview with a Vampire, and that’s not much to go on. I wonder if they’ll pronounce the name Lestrange as Rowling herself does (LESTRAWNGE) or like most of the fans do (LESTRAYNGE).
Anna Friel has definitely said she’d do Tonks – just no one’s asked her yet. Her boytoy, who plays Lupin, is really enthusiastic about the series. After he got the role of Lupin, he installed a “No Muggles Allowed” sign on his front lawn 
Warner Brothers sent out a public casting call for Luna less than a month ago, which means that, like Cho, she shall be played by an unknown. Hopefully they’ll luck out with Luna like they did with Katie Leung, who’s a fine actress for someone her age.
I could see Neeson as Scrimgeour.
I guarantee you Lavender will be in the Half-Blood Prince. Why? Because the pre-teen girl fans obsessed with romance, if they are denied their fix, shall swim to Warner Brothers Studios with machetes in their mouths, and off the studio execs.
Nobody wants to deny the shippers. For them, the romantic stuff in the series is a form of crack cocaine. It also sells a movie.
We had a discussion on a Harry Potter geek board awhile back about what Lavender looks like. She has never been physically described (though I think it says somewhere that she has braces – not sure, though). There is a woman credited as “Lavender Brown” in Prisoner of Azkaban. She was an extra, though, and I don’t believe she has a speaking role. Named extras are an unusual phenomenon, and I’ve never seen it in another series, but there are named extras in the HP series (Millicent Bulstrode is in the first movie, at least).
Lavender was played by a girl named Jennifer Smith, though the IMDB has no picture of her and I’ve never seen her in another movie. There are filographies all over the net that list her in different films, but I think they’re getting confused because Jennifer Smith is a common name. IMDB just has Azkaban.
So far, whenever a background character has become important, rather than teaching the child to act, they simply recast them. There was a Parvati Patil in Prisoner of Azkaban – she turned a snake into a jack-in-the-box – but she was played by Sitara Shah. When they wanted a Parvati who could play a role, they sent out a casting call. Clearly poor Sitara didn’t make the cut, and Shefali Chowdhury was cast instead.
We tried to figure out who Lavender Brown was. She is most frequently identified as a heavyset black girl in the background of Azkaban. I’ve never heard a reliable cite for that, but I suppose she’s as likely a candidate as any. Since Rowling supplies the information about hair colour, eye colour, race, and build to Warner Brothers, then it’s safe to say that even if Smith doesn’t come back for the role, someone similar-looking will be cast.
