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2457308
If you want to see a great example of mirroring, look at that evil guy from Frozen. It's an overrated award-bait garbage pile and Tangled is better, but notice how the evil guy from Frozen... When he's with Peppy Girl, he's cheery and excitable and peppy. When he's with the Duke of Wesselton, he's a serious and stoic and to-the-point. He presents a mask of himself based on the people he's with, not outwardly trying to be an exact copy of them, but instead, something familiar and trustworthy.

I might be giving the writers too much credit here, but in the fairy tale Frozen is based off, wasn't there an evil mirror?

2456713
That's true.
Queen Chrysalis is actually master manipulator, as seen in "A Canterlot Wedding Part 1 and 2", and no doubt many of her changeling drones are quite skillful manipulators too. And "Mirroring" was possibly part of the manipulation tactic that Palpatine/Darth Sidious used to manipulate those like Count Dooku and Anakin to becoming his apprentices.
I really hope that when Queen Chrysalis returns for revenge that it plays out similar to my hypothetical in the video, and that the Sunburst we have been seeing this whole time since the season 6 premier turns out to be a changeling spy in disguise. And with a season 7 changeling character, Thorax's brother Pharynx due to appear, I can see him possibly being manipulated by Queen Chrysalis to join her, and betray his brother, as I mentioned and included him in the hypothetical.

2455865
Huh. You're a more optimistic man than I.

I'm of the opinion that this is just another symptom of the bad writing in so many Glimmer episodes and season 6/7 episodes in general. His awkwardness matches Starlight in near-perfect synch because the writers are idiots that think "Le cringe humor" is the funniest kind, when it's really even lazier than 2000s-era grossout The Splinter humor. Then again, Mirroring is a real-life technique used by manipulators.

2455039
Well, the thing is, Sunburst didn't know about what happened with Starlight Glimmer until she finally told him.

I'm glad you liked this theory. Ever since I saw him as an adult in the season 6 premier, I felt something was right about him. From not recognizing Starlight straight away when Starlight first came to his door, to his awkward, shifty behavior while socializing with her, to not being able to do most magic after many years of studying magic even with his reasonably good potential he had as a colt, to even knowing about Cadance, Shining Armor, and Flurry Heart being taken and replaced, and not accompanying Thorax to go to Starlight for help. All of those things just don't add up.

Plus, Sunburst's backstory still remains a mystery. And when Queen Chrysalis said "There is no revenge you could ever conceive of that will come close to what I will exact upon you one day, Starlight Glimmer!" before retreating, it raises so many questions on it's own. And it can mean she knows something hidden, and how to exact revenge on Starlight Glimmer, and it will possibly have something to do with Sunburst. Kind of like a possible foreshadowing of something we don't know.
That's how I came up with the changeling spy theory.