There's really one overarching rule to being a vice-presidential nominee: Channel your presidential nominee's emotions. If your presidential nominee is sad, be sad, too. If your presidential nominee is mad, be mad, too. If your presidential nominee is happy, be happy. Your job is to be an extension of the top of your ticket, and the best way to do that is to complement, not contradict, what your nominee says and does.

Indiana Gov. Mike Pence (R) is trying really, really hard to abide by that rule. Since presidential nominee Donald Trump announced Pence as his No. 2 a week ago, the two men have appeared several times together in public and given a handful of short, mostly scripted speeches. Pence, who originally endorsed Sen. Ted Cruz (Tex.) for president, seems genuinely excited to be on the ticket. His and Trump's body language suggests they even like each other.

But Friday morning, as Trump and Pence addressed convention volunteers, Pence got a taste of just how difficult his job will be to channel the Trump we've come to know this campaign.

That's because the Trump who most often speaks is the Trump who throws a script out the window, the Trump who makes news almost every time he speaks by saying controversial things, the Trump who promotes himself and insults his enemies ceaselessly, the Trump who gets renounced by members of his own party for, in their words, saying "racist" things.

This was Pence's first real moment standing behind what we'd argue is the more natural Trump. Here's a telling moment about how he reacted to it:

It looks like Pence is trying to do his job and be a good vice-presidential candidate while simultaneously fact-checking his nominee. Right at this moment, Trump was bragging about how the final night of the convention was so popular that tickets for it "were selling for a lot of money on eBay. There were going for big numbers."

Pence gave a polite chuckle, a thin smile, a squint and then, most tellingly, a shake of the head. He was clearly not comfortable with what Trump  just said. 

It's easy to see why. For one, tickets to the Republican convention were not being sold on eBay, according to Politico's quick scan of the commerce site. For two, Trump's eBay comment risks missing the spirit of the convention itself: to celebrate the party, to unify it, to make money, yes, but from donors in high-level suites and not necessarily by selling tickets for it on eBay.

On the scale of Trump controversies, this eBay comment probably doesn't measure up. There have been and most likely will be plenty more things Trump says that traditional politicians like Pence don't agree with or approve of. But Pence's reaction to Trump on Friday is notable — and suggests this is going to be a long 3½ months for him.