With a show that features a lot of time paradoxes, parallel worlds, clones, robot dogs, and other complicated sci-fi elements, it's important that the scripts for each episode are tight, polished, and make as much sense as possible, so that the actors and filmmakers aren't left scratching their heads during shooting.
That being said, quite a lot of TV and movie productions are open to a small bit of improvisation on the day, and that's definitely the case with Doctor Who.
Not that the stars of the show are allowed to make up lengthy dialogue scenes on the spot, but over the last several decades, there are examples of actors improvising short lines or minor actions, those smaller moments that you might not have even noticed.
A lot of the time, these nuggets of improvisation will fit seamlessly into their episodes, and they won't really feel like anything special. But in some cases, they've actually gone on to become highly memorable moments for Doctor Who fans of all ages, proving that, sometimes, the magic of this show isn't always written on the page.
10. Amy's Letter (The Angels Take Manhattan)
This bit of improvisation doesn't physically appear onscreen, but it's such a great story that it's definitely worth talking about.
After Amy and Rory are zapped back in time at the end of The Angels Take Manhattan, the Doctor reads a letter that Amy has written for him. Here, we get an intimate close-up shot of the Doctor's face as he soaks in her final words, with Amy's narration of the letter allowing the audience to hear those words along with him.
While the Doctor is alone in this scene, Matt Smith wasn't alone while shooting it. In fact, Karen Gillan was sat right next to him (just out of frame), reading the letter aloud so that Smith could hear the words in Amy's voice, allowing him to deliver a stronger performance - and this is where the improvisation comes in.
While Gillan was reading the letter, she realised that she didn't have the entire thing with her - she only had the first two pages. As a result, she had to improvise the final chunk, in order to help Smith keep his concentration and finish the scene. This means that Smith's performance is a direct result of Gillan's improvisation. Good job Pond.