My group is, on its majority, formed by young players. Even though we have shared Game Table for 4 years and are close friends of mine, they have a tendency to make shallow characters and observe my NPCs "steal" the spotlight. However I have a player that over develops his character and makes the rest of the PCs look "unimportant", not because he tries to steal spotlight, but because the rest of the guys never have ideas for character development.
How can I encourage my players to make their characters more in depth, speak more than a few lines, and become more active?