Acting for Two: Colin Baker gets to toy with this in "The Wrong Doctors", where you have two versions of the Sixth Doctor 'onscreen' at the same time. Rumor has it they wanted a third, but the studio would have imploded from the awesome.
The Eighth Doctor, played by Paul McGann, constantly gets called a "ponce" and Mistaken for Gay. The Eighth Doctor's second outfit, which made it's debut in Dark Eyes, greatly resembles Paul McGann's outfit in Withnail and I. The leather jacket is a peacoat version of Marwood's jacket.
When Daphne Ashbrook (who played companion Grace in the TV movie) appears, her character immediately decides she wants to shag the Eighth Doctor, with plenty of Refuge in Audacity. A nod to all the controversy over having the Doctor kiss Grace in the first place.
Real-Life Relative: Jake McGann, who voices Alexander David "Alex" Campbell (Eighth Doctor), is the son of Eighth Doctor actor Paul McGann.
Real Life Writes the Plot: The reason Molly was Demoted to Extra in Dark Eyes 3 was because Ruth Bradley had filming commitments in LA and it was thought she would have to record what few lines she did have over there. Turns out she got finished ahead of time, but production had reached the point where the scripts couldn't be rewritten to give her a larger role.
Recursive Adaptation: Big Finish has produced audio versions of the two major Doctor Who stage plays, The Ultimate Adventure (with Colin Baker) and The Seven Keys to Doomsday (with Trevor Martin reprising his stage role as a non-TV Doctor).
Trolling Creator: Nicholas Briggs deliberately trolled the fanbase as much as possible in "Dark Eyes", almost diving into the Last Great Time War for just a few minutes before yanking the idea away again.
The Cliff Hanger between Zagreus' first appearance and the subsequent episode lasted a year and a half. Close to the end of that release gap, in the audio "Omega", Zagreus is prominently listed in the credits. It's an in-story actor playing Zagreus on a Gallifrey-themed cruise ship. He has one line.