When Alphys first brings her script to him, Mettaton is skeptical. The human is pretty clearly still a kid, no matter how determined they are. Is pretending to try to kill them really a good idea?
But for all her fidgeting, Alphys actually looks excited about this plan. After all she's done to help him achieve his dream, it would just be cruel to crush her like this. Even beyond that, they're friends, and he wants to help her recognize her own beauty.
Not that he says any of that, of course. He has no desire to go spend the next five minutes reminding her that he wouldn't be friends with anyone as unglamorous as she seems to think she is. What he says is, "Playing the villain will be a fabulous opportunity to show off my acting ability!"
Alphys beams, and he shoves his misgivings to the side. "Now, darling, let's go over this script. This will be your acting debut, after all, so it has to be perfect!"
It's only his incredible acting skill that lets him keep a straight face as he reads (not that he has a face at the moment, but it's the principle of the thing). The plot is clearly drawn from her anime, from the evil robot to the array of coincidentally-useful gadgets. But this is her project, which he's just acting in. He restricts himself to suggesting minor dialogue improvements.
Eventually they work out something that Alphys is happy with and Mettaton doesn't mind. She runs off to make a copy for him, and he mentally congratulates himself on handling the situation. Then he starts working out how to "attack" the human without killing them.
He's pleased with the final result, given how little time they had. When the human shows up, he's ready with a list of questions and a carefully-nonlethal-laser. Sure, he'd had to wait behind a wall for ages, but his entrance is spectacular enough to make up for it. He's enjoying himself enough that he even takes the chance to poke fun at Alphys. He's just getting into character, so she can hardly complain. Besides, maybe he can finally convince her to make a move.
Finally, Mettaton reaches the end of his questions. As he makes his exit, he glances at the human's face. Fortunately, they seem more entertained than frightened. "They're determined," he thinks as he makes his way to the next scene. "It will be fine." Then he kicks himself ( metaphorically; he still doesn't have legs, sadly). He's the Underground's number one star, so he really should know better than to think things like that.
But despite what his every dramatic instinct tells him, the next scene goes off without a hitch. The human does wonderfully; they don't say much, but they play along nicely and their reactions are magnificent. Alphys has more trouble with her lines, but then this is her first time saying them in front of an audience. Given how she'd acted when she first met the human, the stutter actually contributes to the authenticity of the scene.
Even after everything, he can't help but be impressed by Alphys' jetpack. He takes a moment to consider adding it to the MTT lineup. He could probably convince her to let him sell it. Now isn't the time, though, because the human seems to have gotten over their awe and is ascending the tower.
They make it to the can with no problems, deftly avoiding the ingredients Mettaton is throwing. He has to say, he's rather relieved. Alphys seems to be getting into it now, or at the very least her acting is getting better. He buries his unease under even more enthusiasm and flies off.
His next segment is a news show, much to his disappointment. The news doesn't have nearly enough room for embellishment and style. Everyone just wants to hear the facts, no matter how dull. But Alphys had wanted it, and at least he knew that there would be something exciting to report for once. He can only run so many segments about his own movies before he needs to stop, if only to make more movies.
The human doesn't even appreciate his efforts, instead choosing to report on the present. It's not like their choice actually matters, but he's still disappointed by their lack of taste.
Alphys' call comes through on cue, and the human obediently runs around "defusing" the bombs. Mettaton continues to hover around, periodically commenting on the action. He wouldn't want to deprive his wonderful viewers of any chance to see him, after all.
When the human is done, he reveals the twist. It's an old one, but still not quite a cliche, if only because the reaction is so entertaining. The kid doesn't seem to realize how much work would be involved in cleaning up after a bomb that big.
Alphys calls back immediately. For some reason, she's even more nervous than before, and he ends up having to feed her her lines. He thinks he makes it sound natural enough. The human doesn't seem to notice, at least.
He gives his viewers one last shout out and leaves. He has to say, he's really looking forward to his next performance. He hasn't gotten to do a musical in far too long. He's especially proud of this song; the lyrics are inspired even for him. His dress is, of course, fabulous as well. Even the human won't be able to ignore his radiance this time.
Fortunately for them, they seem suitably awed. He hates to end the performance so soon, but he'd only had time to write the one verse. He makes a note to go back and finish it, then sends the human to the "dungeon". The tile maze wouldn't have been his first choice, but they were running out of preset traps. At least this one runs from Alphys' home computer, unlike the one in Snowdin. That had been a mess.
The human doesn't even make it halfway through the maze. It's hardly surprising, given that they're having to gingerly test each tile to figure out what it does. He's kind of impressed with the way they dodged the piranha. Still, rules are rules, so he activates the firewall.
Alphys hasn't shut it down yet, so he brings it further in. And further. Finally the walls are practically touching the human, and he can't think of any more delaying tactics. He's considering just flying away and letting Alphys deal with it, but finally she turns the flames off. Then she misses her next cue. He's about to take over her lines altogether, but she finally gets herself under control.
Honestly, he loves Alphys, but if she ever wants to do another show together Mettaton will have to give her acting lessons.
With the firewall down, he's free to engage the human. It's just as well that the scene's almost over, because Alphys' stumbles have robbed it of a lot of momentum. Some of his relief probably enters his voice, but by that point he's almost gone. If the human asks, Alphys can deal with it herself.
Unfortunately, there's a large gap between that scene and the next one. He's cancelled the rest of his usual shows to increase the hype, so he ends up sitting around thinking. He's tempted to lie on the ground, but it wouldn't be the same without Blooky.
It's an upsetting line of thought. He quickly starts reviewing his previous scenes, realizing too late that that is also not ideal. Now that he's thinking about it, he can notice what he was trying to avoid noticing: how much the human relied on Alphys. Sure, he wouldn't expect a kid to be able to defuse a bomb, but they hadn't even moved when he came at them with a chainsaw.
He shouldn't care, not as long as they play their part, but as unattractive as it is he's dwelling. He decides to change the script, just a little. He'll change the layout of the Core just enough to make them figure it out themself. Then Alphys can come in and save them, in the process realizing her own worth, and Mettaton can go back to entertaining people without all these pesky moral dilemmas.
He makes the call to rearrange things, but for some reason he still can't relax. Watching the human doing puzzles doesn't help; it's just boring. He needs to make things more exciting. After all, he needs the tension to be suitably high when they get to the finale.
He makes another phone call. The human will need to learn how to fight anyway if they want to get to the surface. Really, he's doing them a favor.
Or he would be if the human would cooperate. For some reason, they've chosen now to be contrary. They keep finding ways to get past his mercenaries without fighting. Honestly, they got through a knight by praying? If he had time, he'd record a CD like the one he made for Burgerpants.
But there isn't any time, the human is right outside. Mettaton makes a decision. He's going to write his own script, one in which he makes it to the surface. He'll be ten times the star he was in the Underground, and no one will die except the human.
It'll be worth it. Alphys will understand.
He puts on his cheeriest, most enthusiastic tone to explain everything. If he's just playing a part, he doesn't have to think about what he's doing. Besides, the human deserves to go out with as much style as possible.
For once, Alphys manages to call right on cue. She seems to have finally gotten into character, in the process forgetting that Mettaton can hear her over the phone. He almost laughs, but he decides to play along. He's been waiting for this body for so long, though he'd hardly imagined he'd get it like this. At least this way he can make sure the human's death is beautiful.
He turns around with a mixture of eagerness and dread, and waits for the human to flip his switch.