Characters have the *ability* to be bought as you go - but you do have to actually pay for them when you do :) This unlocking is separate from characters you unlock during gameplay.

Unlocked by end of Episode IV
gonk droid 1550
greedo 60k
sand 14k
jawa 24k
tusken raider 23k
imperial shuttle pilot 29k
stormtrooper 10k
rebel trooper 10k
imperial spy 13500
beach trop 20k
death star trooper 19k
tie fighter pilot 21k
imperial officer 28k
grand moff tarkin 38k (what's with that name? And if there's a Grand Moff, is there a regular Moff?)
tie fighter 30k
tie interceptor 40k
tie fighter darth vader 50k

Unlocked by end of Episode V
han solo hood 20k
rebel trooper hoth 16k
rebel pilot 15k
snowtrooper 16k
lukeskywalker hoth 14k
tie bomber 60k
imperial shuttle 25k
lobot 11k
Ugnaught 36k
bespin guard 15k
princess leia prisoner 22k

Unlocked by end of Episode VI
ewok 34k
imp guard 45k
emperor 275k
admiral ackbar 33k
ben kenobi ghost 1.1 mil
anakin skywalker ghost 1 mil
yoda ghost 1.2 mil
ig-88 100k
skiff guard 12k
boba fett 175k
4-LOM 45k
gamorrean guard 40k
bib fortuna 16k
palace guard 14k
bossk 75k
densar 70k

Lego Star Wars 2 Walkthrough