To best understand this animated analysis of Bach's Fugue V, WTC BK. I please listen to Ann discuss it on YouTube.

...but if you are in a hurry, just watch the subject repeat itself here in all the colors and imagine you are playing a game where the rule is to know how to sing a melody in order to play the game.You must sing the subject in order to enter the game. That is how the voices enter in a Bach fugue.

Pianist: Ann Cummings
Recorded at Lake Union Recording Studio 2010
Sound Engineer: David Loy
Animation: Flash 5, Ann Cummings 2011