Although the Doctor often pretends to have a plan, they are, more often than not, making things up as they go along.
Nowhere is the show more upfront about this than in Legend of the Sea Devils, when the Doctor tells Yaz that she intends to use a "beautiful bit of improvisation” to save the day.
Of course, behind the scenes, things are much more straightforward, with scripts to work to and schedules to follow. But if you think that means there’s no wiggle room, think again.
On the contrary, there’s always been plenty of opportunity for actors to bring their own suggestions to the table. And though the decision of whether or not to include such ad-libs ultimately rests with those in charge, it’s only right that the actors are given the chance to try things out. After all, they know their characters better than almost anyone else.
This insight has resulted in some real gems over the years – many of which feel so natural, that you would never have guessed they weren’t scripted.
10. “It's Bigger On The Inside!”
In The Husbands of River Song, the Twelfth Doctor makes a big speech about the TARDIS being bigger on the inside, pretending to be an oblivious newbie whose “entire understanding of physical space has been transformed!"
It's a glorious moment, but this wasn’t the first time the stock response to the TARDIS' dimensions had been played for laughs.
Following a fairly standard “bigger on the inside” scene in Rose, Smith and Jones presented the crew with an opportunity to have a bit more fun.
Hence why – when Martha enters the TARDIS at the end of the episode and speaks that iconic line – the Doctor mouths along, before flippantly responding, “Is it? I hadn’t noticed!”.
Although the dialogue here was scripted, the Doctor miming along with Martha's "bigger on the inside" was an ad-lib by David Tennant.
It’s a funny moment in its own right, but it's also totally in keeping with the more aloof, sarcastic characterisation of the Doctor in Series 3, following the loss of Rose.
This is a Doctor who has much less time for prospective companions than usual, yet still finds himself showing off. Even when no-one’s looking!