Complex sentence: Meisner taught actors to achieve spontaneity by understanding the given circumstances of the scene (as did Strasberg) and through interpersonal exercises he designed to help actors invest emotionally in the scene, freeing them to react "honestly" as the character.
Split into simpler sentences: Meisner taught actors to achieve spontaneity by understanding the given circumstances of the scene (as did Strasberg). He designed interpersonal exercises to help actors invest emotionally in the scene, freeing them to react "honestly" as the character.