Speak to Aristophanes in Perikles's residence. His plan to weaken Kleon's hold on Athens is to create a comedy to embarrass him. Problem is, he's missing his star actor. In the conversation with him, Gather information about Thespis.
After this conversation, leave Perikles's Residence. Head to Thespis's house which is in Greater Athens near the Agora. Call Ikaros when at the Agora and look across the small river running through this part of the city. An Athenian guard will be rummaging through Thespis's belongings.
Gather information from the guard. Threaten him, have him join the Adestria, or let him go. This choice will not have long-term effects. If he joins the crew, he will join as a new lieutenant.
From there, Head to the tavern fairly close by. From Thespis's house, go left and down the stairs. At the end of the stairs, head right. Take another right at the next path with a small set of stairs. Follow the path down and turn left into the tavern.
Speak to Thespis. He's sitting and rambling at a table surrounded by Athenian guards in an orange robe. The officers are reluctant to let him go. Threaten them, let them go, or pay them.
Choosing to pay them will cost 600 drachmae, but it's simple and combat free. After the guards are satisfied, carry Thespis back to his house.
Thespis refuses to perform without his muse in attendance. After this (pathetic) conversation with him, the mission will come to an end.