Why do Frodo and other characters that don't appear in The Hobbit appear in the cast of Peter Jackson's film?
Frodo turns 33 when Bilbo turns 111, so Frodo was born the day Bilbo turned 78. In FOTR, it is said that Frodo was becoming restless when he turned 50, and that it was near that age that adventure had befallen Bilbo. This means the events of The Hobbit happened twenty-something years before Frodo was born.
The appearance of Legolas, Galadriel, old Bilbo and Saruman suggests scenes "from the future", but the appearance of Thranduil suggests that Legolas might be playing a major role in the movie.