'Hawaii Five-O' casts Alan Ritchson of 'Smallville' and 'Blue Mountain State' as McGarrett's brother-in-arms

"Blue Mountain State" star Alan Ritchson spent some time in Hawaii last year, filming scenes for "The Hunger Games: Catching Fire" -- and now he's returning to the Aloha State for some time with Danny and McGarrett.

Ritchson has nabbed a guest-starring role on CBS' "Hawaii Five-0," and Zap2it has your exclusive details. He plays Freddie, described as a sweet guy who is built like a fullback. Freddie is a Navy SEAL -- and he was McGarrett's best buddy during his years in the military.

We'll see them in flashbacks to their time training, when they supported each other through some extremely difficult conditions, and later, in battle together, when they had to make some painful choices together.

Ritchson is best known for his work as Thad Castle on "Blue Mountain State" and as "Smallville's" Aquaman. He'll appear this fall in "Catching Fire" as Gloss, one of the career tributes in the Quarter Quell Hunger Games.