Play Tip: The majority of the Lego The Lord of the Rings characters are unlocked from finishing story missions (the Story Mode Levels). Some are just fully unlocked from completing missions, others are made available to purchase with studs. Some characters you could otherwise buy, can be unlocked via cheat codes or via Middle-earth Characters Pack downloadable content (DLC)*. All of which are listed below.
It appears to me that most characters are unlocked after completing story missions, purchased with studs, or found wandering in the wilderness. There are cheat codes, but honestly what's the point of playing a game that way. The DLC's probably not up yet as the game is still just rolling out.