ON ACTING: Preparation and Performance
To differentiate them most simply, chronology is most useful: Preparation is all that the actor does prior to performance (pre-performance); while all that the actor (as character) does onstage or--after the actor has been transformed (by preparation) from the actor into the character--can be seen as the actor's performance.
Rehearsal and preparation comprises the actor’s pre-performance activity, all that occurs in anticipation of performance: learning lines, finding the right costume, analyzing character, rehearsal, etc. (A warning-on-the-label: An actor cannot rehearse a future performance; all s/he can do is to rehearse for a performance. Preparation pre-pares us for something that has yet to happen in reality. I often use the sports analogy: you can’t practice a game itself; you can only practice for a game. The game has yet to happen. )
Granted, in rehearsal, tendencies of the opponent—what I would call expectant reality--can be prepared for: character anticipated leanings, inclinations, predispositions can be anticipated, but in reality the future (of the other performances as well as yours) can never be fully known.
The final upcoming onstage or onscreen performance is fundamentally unknowable. It will (and must) flow freely in the performance itself: albeit within the narrowest confines of its actor and director pre-designed (pre-anticipated) banks.
A performance is fundamentally improvisatory. Therefore, the need for true reality requires, nay demands an actor’s anticipated flexibility when anticipating or preparing for any subsequent performance. Humans/actors must accept they—life--are not machines; their precise actions in the future cannot be anticipated and replicated in performance from even after the most assiduously prepared design. The banks of any future performance river can be precisely molded; but the actual running of the river, the flow of human emotions, is deep, complex and ultimately unknowable, and will ultimately flow as they will. An actor’s attempt to do otherwise will fail. The rule: Prepare minutely and thoroughly; then perform freely. They are two distinct stages, two different realities...but...preparation may lead to performance, but preparation never equates with performance.