Play throughout the game to unlock characters to buy in Freeplay mode:

Amidala - Complete Level 3 (Naboo City) in Episode I
Anakin - Complete level 1 of Episode II
Chewbacca - Complete Level 2 in Episode III
Darth - Maul Complete Episode I in Story Mode
Darth Vader - Complete Episode III in Story Mode
Dooku vComplete Level 1 in Episode III
Jango Fett - Complete Episode I, II, and III
Jar Jar - Complete Level 2 in Episode I
Qui Gon - Complete Level 1 in Episode I
R4-P17 - Complete level 3 in Episode III
Yoda - Complete Episode II in Story Mode