Why? Wishing anyone, particularly an actor, "Good Luck" is apt to fill them with confidence -- feeling as though they have "luck" on their side.

Confidence in an actor can quickly lead to castasrophy, as it causes them to lose focus.

"Break A Leg" is a very old military term for "taking a knee", or bending down to one knee and breaking the line of the leg. In the theatre it is a reference to "taking a bow". To wish someone to "Break A Leg" is to ask them to give the best performance they are capable so that they may deserve to take a bow at performance end or, to "Break A Leg".

Another reference to "break a leg" is to slightly bending one's knees. Nervousness and adrenaline can make an actor "lock" their knees, which makes them look stiff on stage and can lead to feelings of illness under the hot lights and even cause them to pass-out. “Break a leg" can also be applied as a reminder to relax, loosen up, and enjoy giving the performance.