This happens verrrrrrrrrrrrrrry rarely - indeed I can count on one hand the times when the movie or other production has been as good as the material it was adapted from....(and that includes Peter Jackson's Lord of the Rings debacle - why the heck did he dismiss Lee's Saruman so stupidly?????)
But one such adaption is The AMazing Maurice and His Educated Rodents. The BBC adapted it to radio in 2003, and BBC 7, which I listen to, broadcast it in June 2007, which is when I recorded it... but I didn't have time to listen to it until a few days ago.
And I love it. The voices fit the cast perfectly, including David Tennant as Dangerous Beans (Tennant is the new Doctor Who) and Shaun Prendergast as Darktan, and frankly I like the adaption much better than the book - which I read for the first time today.
Maurice is a cat who lived around the garbage heap behind Unseen University - a place where wizards lived. Some of their magic escaped and gave Maurice intelligence...and it also gave intelligence to a lot of rats - now called The Clan.
The Clan wants to find an island where they can live as civilized beings, away from humans. Their leader is Hamnpork, an old rat who isn't in tune with new ways, their spiritual leader is Dangerous Beans, who has read a book called Mr Bunnsy Has An Adventure - about humans and rats interacting, which he regards as true, and therefore, as his Bible, and various others.
Maurice has recruited Keith - the "stupid looking kid", and in order to get the money to buy their island, they go from town to town running the "Plague of Rats" caper. The rats invade, Keith comes in with his pipe and for a fee, plays music that causes the rats to follow him out of town.
Then they come to Bad Blintz. Where things are really, really bad.
The adaption leaves out much (it's only 90 minutes long) and adds a little. My favorite scene, in which Darktan dismantles a trap while new-to-the-trap-squad Nourishing looks on, is great on the radio, not so great in the book.
The only flaw in the radio adaption is a slignt one - the voice of Death, at the end. Death is one of my favorite characters in the Discworld series of Terry Pratchett (indeed, I prefer to think of that Death as Death in the real world, too, very comforting), but in the adaption, in his few lines, he comes across as way too.... avuncular and smarmy...at the very least the voice should have been put into an echo chamber of some kind so you'd know it was Death talking...
BBC Radio 7 repeats things all the time, so make sure you check back periodically for The Amazing Maurice and his educated Rodents, although I'll also let you know when it's being repeated again, here.