Question: who played role of joker in dark knight
[{'answer': 'Heathcliff Andrew Ledger'}, {'provenance': [{'wikipedia_id': '17173785', 'title': 'Heath Ledger', 'start_paragraph_id': 1, 'start_character': 36, 'end_paragraph_id': 1, 'end_character': 726, 'bleu_score': 0.8821117887220441, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15373111016662512936}, 'section': 'Section::::Abstract.'}]}]