After you beat a level, you automatically unlock free play. Free play allows you to use any characters you have unlocked in that level. You can choose any character that you want to use in the level. Small characters (young Boba and young Anakin from original and Wicket from 2) can crawl through small gray doors. In flying levels (Hoth battle, episode four death star, episode six death star) every ship is available during the level.
Note: Only a certain number of characters are available to use.
Note 2: If you have use old save, only one of the original characters will be on your team. You have to choose the character that you want to use. A good idea for this would be to turn on player 2. You can then choose two characters to use from lego star wars. After you choose, turn off player two so you can move freely.
Note 3: You can use any character that you have unlocked.