Mark Strickson is at his tastefully over-the-top best in this episode, denying with vigour that he is spying on Wrack, and giving the most intense stare ever seen on television when convincing Wrack that he, too, serves the Black Guardian, and that he wishes to serve her. Judging by his earlier actions in suggesting that The Doctor be banished back to Striker's ship, you could believe that Turlough is bluffing when he tries to charm Wrack.
Turlough has been conflicted for some time, though, even during the climactic meeting between he and the White and Black Guardians. Strickson again is stellar as he mulls over his choice between giving the Black Guardian the diamond or The Doctor. Look at The Doctor's face, though, while Turlough is deciding - it is a face of calm, knowing strength. The Doctor knows what Turlough will decide, almost as if he has known what he will decide for some time, ever since he found out that Turlough was a servant of the Black Guardian in Mawdryn Undead. Back then, The Doctor could have tried to put a stop to the contract between Turlough and his master, but he didn't. He knew he had to see it through to the end, to this very moment on Wrack's ship when Turlough had to make his choice. And he knew that Turlough would make the right choice.
It is often said that the Fifth Doctor is a failure as a character, that he was made much more vulnerable than is predecessors. He is anything but a failure, in my mind. Perhaps more than any other Doctor, the Fifth Doctor makes others around him better people. He turned Adric from a whiny teenager into a noble hero who died trying to save others. He turned Nyssa from a privileged daughter of a future Keeper of Traken to a brave, courageous person who risked her life to help the Vanir treat Lazars Disease on Terminus. And, perhaps his most impressive achievement, he's convinced Turlough that he's a better person than he thought he was, and that Turlough would do much better in life to join him in his travels instead of killing him. The Doctor has set the template for Turlough to be a strong companion for the rest of the boy's time in the TARDIS.
Enlightenment is an exquisite example of Doctor Who at its very best. It is full of rich imagery and deep, well thought out characters who are all seeking one thing - Enlightenment - but each character has an entirely different interpretation of it. Wrack wants power, Turlough wants freedom, while Striker is delightfully enigmatic and doesn't divulge his desire. The one person who wins the race is The Doctor, and he is the one person who doesn't want it. Only a true, enlightened man such as The Doctor would know enough to not want such a prize.