That's what I imagined Twilight telling her.
The episode had its highlights, but they were quite timid and enjoyed best without pointing them all out.
In the beginning, Celestia thinks theater brings out the best in people. Well, dunno about Equestria, but in our realm there often seems to be lots of jealousy, competitive friction and big egoes involved.
When Pinkie showed she counts the times AJ told her not to do something, I thought, well, that shows she cares, hehe.
When Celestia was whispering her lines, I thought: She really needs Rainbow Dash for voice coaching. (LOUDEEEER!!!!)
The plot was very predictable early-on, and although I foresaw awkwardness, I didn't expect the execution to be THAT bad, and it really didn't have to be. (But sense for the power of subtlety seems to be an art.) In a way, the intriguing point about this episode was that it had a theme of really bad acting while the episode itself has a badly executed script.
The lesson of telling the truth is really getting old, and while I understand the pressures that made Twilight make the mistake yet again, Celestia said at one point: "Didn't Applejack remind you that telling the truth is always better than a well-meant lie?" ... Hmm, didn't we have an episode involving AJ that conveyed something slightly different? About sparing someone's feelings?
And Celestia really has to try hard to be that bad at acting, especially since she just needs to be herself. For her sake, she's a thousand years old. One should pick up a thing or two. During theater lessons she's outright acting like she has a mental handicap. All very forced and exaggerated. That's so not her. Someone who is excited about theater would love to play pretend, and it got even weirder when she suddenly started giving Fluttershy acting tips. How can someone who is so passionate about something be so horrible at it, and how can someone so horrible at it be able to teach it? That seems unnatural and contradictory. Then in the end she must have unlocked an achievement or something, because suddenly she could act. But there wasn't really anything preventing her from utilizing that passion before for the stage play.
Pinkie's unaware-ironic fireworks line then was as clumsy as it gets. Again, really bad acting, hah. It wasn't even delivered in Pinkie's usual way where SHE likes to play the dramas of life.
P.S. (Pinkie scriptum): The balloon WAS the perfect substitute for the sun? Don't tell me Pinkie only has ONE giant yellow balloon in stock! I refuse to believe that! What kind of world is that supposed to be?!
UPDATE: When re-watching parts of the episode, I noticed someone is really not amused by this interruption of the night. "If my coat is getting pale, I'm gonna give YOU a tar coat!" - Damn cheaters, right?