If you play as Gideon, you'll eventually reach a part of the game where you won't have Rachel with you. But if you use the lockpick to unlock a door, you still hear Rachel's voice say, "That worked!", even though she's not there. The effect of this glitch (or oversight) is reversed if you chose to play as Rachel instead. You'll hear Gideon say, "That worked!", even though he's no longer with you.
Taken with permission from Flying Omelette.