For those who are curious as to how Peter would be as a companion...

CONTEXT
The Doctor (11): Karen Gillan
The Doctor (12): Jenna Coleman
Peter Capaldi: Peter Capaldi
Matthias Lake: Matt Smith
Episode: The Time of the Doctor
Warning: Swearing (censored *)
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Peter watched with fear as the Doctor suddenly turned around and looked at something, but when he himself does, he saw nothing there.

To the Doctor, however, her TARDIS walls were filled with lots of colourful drawings and the room gave off a soft, orange glow, resembling the one the previous TARDIS interior had. At the corner of her eye, a young boy ran along the rails, a piece of paper in his hands. He tripped slightly but continued moving. She can't see his face, but she knew who it is.

"Matthias?"

Peter was old, yes, but why wasn't he the one hallucinating?

Gulping, the man asked softly, "Matthias who?"

The Doctor shifted her vision to him, movements slow and gentle. She gave him a warm smile and pointed at the child, "The first face this face saw."

Peter scanned the TARDIS once more to confirm that he was the sane one. Nothing's changed.

The Doctor noticed his panicky state. She knew this was scary for the both of them, but it couldn't be stopped. It was her nature, and his, and he might just leave her after she changed.  She doesn't want to leave him just yet, they've only just met, future her was going to need him more than ever. She attempted to calm him down the way she always did.

"We all change, when you think about it," she started, earning her companion's attention, "We're all different people all through our lives. And that's okay, that's good, you've got to keep moving," she did a rolling action with her hands, "so long as you remember all the people that you used to be."

The problem was, to Peter, was that this was how he wanted to remember her. He can't imagine another Doctor that he has to run with. He can't imagine another Doctor that he can pour his heart and soul into. What if the next one was short? He'll have to bend down and talk to her and god knows that that's bad for his back. What if the next one hated him and didn't want to travel with him anymore? She might just dump him back home and forget he ever existed. Consumed in his thoughts, Peter didn't make a move or sound. He focused back on the Doctor, vision becoming slightly blurry.

"I will not forget one line of this," The Doctor promised, "Not one day. I swear. I will always remember when the Doctor was me."

The Doctor turned back to the stairs. Her heart melted. He made a promise and he kept it. He was there, till the end of her.

Matthias Lake placed one hand on the railings and the other one in his pocket. He grinned at her and winked, and she can't help but let out a short laugh. His face hasn't changed since the last time she saw him, and he was wearing the same outfit he always does. Nonetheless, this was merely in her mind. Peter couldn't see any of this.

He took slow steps down the stairs, eyes fixated on her. His actions were exaggerated, just as dramatic as before. The Doctor smiled, watching him and his silly self. Finally, he reached the bottom and touched her cheek. She did the same, and swore she could feel him. His warm hand gave her comfort, just like before.

"Time to go, ginger," Matthias said softly, as if luring her to join him.

"Good night."

She blinked, and he's gone. It was time.

The Doctor looked back at Peter, who had made no movement at all. He was probably confused, and scared, and how she wished to pull him into a tight hug but she can't.

With a pained expression on her face, the Doctor removed the yellow bow on her head and let go of it. It fell gracefully to the ground, curling as it landed. She's going to miss this, all of this.

Witnessing what the Doctor was doing, Peter came to his senses and stepped forward. It's happening, and he wanted her to stop.

"No," he begged, "No, please. I'm breaking your rule and I'm going to swear if you change."

"Hey..." she mumbled, energy draining as the regeneration was about to commence.

"Why aren't you stopping?" The companion offered his hand, hoping to touch the Doctor one last time, "Don't you change. Don't you dare f*cking change."

The Doctor did the same, yet her body leaned back further. Her arms started glowing brighter and brighter.

"Please don't change," Peter pleaded, "Stay with me."

She didn't want to hurt him, seeing as she's about to do more bad than good by changing. She stretched, and their fingers were just centimetres apart. Just a little further...

It all happened too quickly. Her head snapped back and when she was upright again, Peter didn't recognise her.

The duo stared hard at each other, none making a move.

Her hair had turned black and was of a shorter length. Her face was rounder and the Scottish element was no longer there. Her clothes dangled of from her, she clearly shrunk. Her eyes were much bigger and her nose seemed a little out of place. The worst part of all, his worst fear had come true. She was shorter.

There was only one word on his mind.

F*ck.

She blinked, breaking the staring contest. Suddenly inhaling, she winced and grabbed the side of her abdomen, stumbling backwards.

"Kidneys!" She exclaimed, giving Peter quite a shock, "I've got new kidneys. I don't like the colour."

The old man's eyes were as wide as saucers, having not come to terms with the new woman in the TARDIS. Even her accent has changed, she wasn't Scottish!

"Of your kidneys?" He asked, "They're just brown!"

A sharp jerk of the TARDIS caused the both of them to lose their balance. Instinctively, they grab hold of the console, escaping the possibility of falling.

He blurted out suddenly, for he couldn't stand the thought of it, "You're shorter!"

"Oi!" The unfamiliar woman said, "Rude!"

The TARDIS started vibrating before commencing to lurch from one side to the other, throwing the two around the console. Clinging onto a lever, Peter steadied himself and asked, "What's happening?"

With that, he swung the console screen to her.

The new Doctor struggled catch the monitor, almost missing it by a finger. A moment of stillness, and she positioned the screen in front of her, scanning over what was written on it.

"We're probably crashing. Oh!"

"That's helpful! Into what?" Peter shouted over the noise that the TARDIS was making as it lurched to the side again.

The Doctor pushed the screen away and remarked casually, "Stay calm."

That was kind of hard to do.

"Stay calm?" Peter reiterated, anger and fear building up in him, "How the f*ck do you expect me to stay the f*ck calm? We're dying and crashing into who knows what! Don't you have any f*cking green and blue buttons to push?"

She took a quick glance at the console before looking up at her companion and smiling sheepishly, "Yeah! Just one question."

"You should be giving me the f*cking answers!"

"About that," The Doctor was to a wall and caught herself with the railings, "Do you happen to know how to fly this thing?"

A moment of silence, and Peter threw his hands in the air. He didn't give a f*ck about the swearing rules anymore.

"We're so f*cked. I'm having SUCH a great f*cking day today!"

And the TARDIS spun out of control again.