Fourteenth Doctor actor David Tennant shared a curious hint about the casting in the second Doctor Who 60th anniversary episode Wild Blue Yonder to Total TV Guide magazine.

“The second script is unlike any Doctor Who I’ve ever seen. I don’t want to give too much away, but I was always aware of the casting that was going to be part of the episode. That was something that [showrunner] Russell [T Davies] was very specific about. I just didn’t imagine it would… manifest itself in the way that it does…”

When casting is specific, it often means the casting director is looking for a specific physical type or special ability. Thanks to promotional images, we know that the special features at least one robot. However, casting specific types of actors to embody robots is not unusual to Doctor Who casting — so it remains unclear why Tennant was surprised by how specific types of actors were used in Wild Blue Yonder.

Another possibility is that the actors hired are large and tall. A still from one of the teaser trailers and a promotional image show the Doctor dwarfed by a very large Chair.

The only known cast member in the special besides David Tennant and Catherine Tate is Susan Twist, who plays a character named Mrs Merridew.

The Wild Blue Yonder Synopsis:

The TARDIS takes the Doctor and Donna to the furthest edge of adventure. To escape, they must face the most desperate fight of their lives, with the fate of the universe at stake.

YouTube teaser trailer:

Two scenes from Wild Blue Yonder:

@BBC iPlayerBrand new Doctor Who. Continues 2nd Dec on BBC iPlayer.♬ Promoted Music – BBC iPlayer

In addition to the teaser released on YouTube, another teaser was released on BBC iPlayer which showed the Doctor and Donna looking at files with the Doctor deciding to taste one of the very slimy files after assuming they aren’t dangerous. The clips ends just as the Doctor begins to gag.

There are many released images and stills from various trailers for Wild Blue Yonder, the most mysterious of the three Doctor Who 60th anniversary specials:

Written by Russell T Davies and directed by Tom Kingsley, Wild Blue Yonder, the second of three Doctor Who 60th anniversary specials starring David Tennant as the Doctor and Catherine Tate as Donna Noble, will be available on 2 December at 6:30pm exclusively on BBC One and iPlayer in the UK and Ireland and exclusively on Disney+ everywhere else on the same date.