The principal function of the actors was to carry on the dialogue and work out the action of the play. The principal function of the chorus was to sing the odes which filled up the pauses in the action. Of course very frequently the chorus took part in the dialogue; but, speaking in general terms, the dialogue was the business of the actors. Such was the condition of things during the best period of the Attic drama. But in former times the case had been very different. At first the whole performance was a choral one, and consisted simply of the songs and hymns chanted at the festivals of Dionysus. There were no actors and there was no dialogue. The history of the early development of the drama is in other words the history of the gradual introduction of actors and dialogue into a choral entertainment, and the gradual increase in the importance of the dialogue, until eventually it overshadowed the choral part altogether. The first step in the process by which a lyrical performance was converted into a dramatic one was as follows. The custom arose of filling up the intervals between the different portions of the choral songs with recitations by the leader of the chorus, and dialogues between him and the other members. For this purpose the leader of the chorus used to mount upon a small table. The subject of the recitations and the dialogues would be the same as the subject of the ode, and would in most cases refer to the adventures of the god Dionysus. In these interludes by the leader of the chorus lay the germ of the drama. The performance as a whole was still essentially lyrical, but the practice of inserting dialogue had been established.[670] In the case of tragedy the next step forward was taken by Thespis. He introduced a single actor, who took the part which had previously been taken by the leader of the chorus, and filled up the pauses in the choral odes either with monologues or with dialogues between himself and the leader.[671] Not much is known about the drama of Thespis except that it was still essentially lyrical. But as he is said to have employed masks, it is clear that the single actor might appear in different characters in successive scenes, and in this way some approach might be made to a dramatic representation of a story.[672] The decisive innovation was due to Aeschylus. He introduced a second actor, and effected a total change in the character of the performance. Henceforward the intervals between the choral odes were filled with dialogues between the two actors upon the stage, instead of dialogues between the single actor and the leader of the chorus. At the same time Aeschylus cut down the length of the choral odes, and made the dialogue the essential and prominent feature of the performance.[673] The result was a radical change in the nature of tragedy: it became a dramatic instead of a lyrical form of art. During the greater part of his career Aeschylus was contented with two actors.