Old Year! upon the Stage of Time You stand to bow your last adieu; (lines 9-10)
This kicks off the big theater metaphor. We feel like we have to make a big deal out of it, because it takes over so much of the poem. Right here it just looks like a toss-off, something he's going to mention and then move past. Ha! Service has much bigger plans. He's going to put us in that theater. He's going to make us look at the people sitting around us. This is a metaphor on steroids, and it really turns into the life of the poem.
A moment, and the prompter's chime Will ring the curtain down on you. (lines 11-12)
In the past, theaters used a person called a prompter to help run things backstage. Apparently it was his job to ring a bell to signal the lowering of the curtain at the very end. This is important, because it really enriches the theater metaphor, giving us a stronger sense of really being there in this spooky, imaginary audience.
And face your audience again. (line 16)
Now it seems like the Old Year is coming back out for an encore – our speaker really just can't let him go. This whole idea of facing the audience is interesting. When you come out for a bow, you're giving the audience a chance to thank you, to make you feel good. Here, though, it sounds like he's going to face a firing squad. In fact, the audience doesn't seem all that thrilled – two thirds of them are crying or hiding in the shadows. In any case, combining poetry and theater in this metaphor helps to dramatize this moment. It lets us visualize the abstract idea of the passing of the year.