Choice means, strangely enough, exactly that: the choice an actor makes at each moment. These will be different for each actor and each piece.
When Geoffrey Rush played Sade, he chose to think of a goat, with all that implies in physicality and movement (not to mention the classic association of a goat with lust.)
Or take "Hamlet". An actor could decide that Hamlet has an incestuous desire for his mother (I've seen at least one production with this made apparent - but the director might also have imposed that choice on the actor.) Or that Ophelia is pregnant with Hamlet's child. Or that she has refused all his advances. The actor playing Ophelia on the other hand may decide she thinks of Hamlet as a brother. Or that she is afraid of him.
Each of these choices will give a very different tone to the performances. Another standard choice for the actor playing Hamlet to make is, is he really mad, or is he feigning madness?
Similar choices can be made about almost anything - what does Hamlet smell when he comes into a room? Flowers? A rotting corpse? Nothing? Some actors will make no choice about that at all. Others may make the choice but without it being in the least apparent to the spectators.
Has Hamlet ever killed anyone? Given that he engages in a duel to the death, that would inform his ability to do it - for the first time or just one more time.
Etc. Choices can be about general circumstances, motivation, physicality... Almost anything. Even the absence of a choice is decisive to a certain degree. My own guess is that actors who make no specific choices at all are boring to watch. But there might be exceptions. And again, some choices are clear to the spectator, some inflect the performance without being obvious in themselves, some may not, ultimately, affect the performance at all. But in every case, the actor, however consciously or not, has chosen to go one way rather than another.
The word, simply put, means what it does in plain English.