Sometimes when a series adds a major new character late in the game, it's a sign of desperation (see Leonardo DiCaprio's stint on "Growing Pains"). But sometimes, the casting is just right. Everybody loves John Lithgow, even people who can't quite remember his name. But "Dexter" fans everywhere surely swooned when it was announced that he was joining the cast as a BTK-style serial killer for Season 4. Meanwhile, "24" fans met the news that "Scooby Doo" star Freddie Prinze Jr. was joining their show with a collective shrug. No one can compete with the Bauer! No one!