Consider the following:

(Show) Cassidy slammed her notebook shut and stood up, taking a step back from her advisor. "This is the second and last time I'm going to tell you: what you're doing is considered sexual harrassment. I'm requesting a different advisor if you don't stop what you're doing and start advising me."

"Cassidy, what's gotten into you?" He patted Cassidy's shoulder. "I'm only trying to help."

and:

(Tell) Cassidy was fed up with her sleazy advisor's advances. She set an ultimatum, allowing him one last chance to start advising her as he was supposed to before she reported him.

It's pretty clear here that the 'show' example is the better one. It takes up more space, true, but it lets us know a lot more about both Cassidy - she's pretty tolerant and her advisor may think that his behaviour is, in fact, acceptable. And I didn't even have to use the words 'sleazy' or 'fed up' for you to know that they applied.

On the other hand, there is this:

(Tell) They had been searching for 5 hours, pulling encyclopedias, reference books, anything that looked like it might have something for them off the shelves. The dust danced in the light from the overhead lights, another puff jumping into the air with every book from the back room they opened.

You really don't want to 'show' something like that, do you? It would be mind-numbing. A sentence or two does an admirable job of telling the reader what's been going on and lets you get on with the business of showing the important stuff. In this case, 'telling' would be the preferred method. You could add in a bit more description or perhaps name a few of the books they look through, depending on how important that 5 hour search is to the story, but should only do it as a 'show' scene if it is absolutely critical to the plot.

However, what about the following:

(Show) The sun was approaching its peak and they slowed to a walk. Kerry started looking from side to side, searching for a reasonably comfortable spot to spend the hottest part of the day.

"You're being paranoid, you know," he commented when Nancy followed her brother towards a likely-looking break in the undergrowth. "It's been weeks. I'm sure they've stopped looking for us by now."

Nancy paused, and glanced back at Kerry. "Perhaps, but I won't feel safe until I've rejoined my family."

He rolled his eyes. "All right, but don't expect me to help you get the sap off."

"You should hide a bit better than that," Nancy added. "Lying on the side of the road just screams 'come attack me, all you bandits and highwaymen!' - even if they aren't the ones we escaped from."

Kerry smiled and nodded indulgently.

"I don't think you understand." Nancy stopped just short of the edge of the road. "I've been travelling a lot longer than you have, and the bandits know nobody travels alone. They'll see you, and start searching for us."

"They can't afford the time. There's only so long they can go between raids if they want to keep eating." Kerry leaned against a tree and closed his eyes.

He didn't seem willing to continue the argument, so Nancy hurried to catch up to her brother, and they started searching for a good, hidden place to rest.

Maybe it was paranoia, but none of the hiding places they would have chosen only two months ago looked hidden enough. Nancy sighed and looked up into one of the biggest trees in the area.

"Not again," her brother complained.

"Did you see a better spot?" Nancy replied. "Come on." She struggled up the branches, fighting against the twiglets that grabbed at her and reaching for the thickest branches to pull herself higher. She paused for breath and looked around. Above her, on all sides, and below, were branches and leaves. She carefully shifted to a pair of branches at the same level that looked like they would be strong enough for her to sit on.

Or:

(Tell) They had fallen into the habit of taking their midday rest off the road and well hidden. Kerry said that they were paranoid and it had been weeks since they had left, but he didn't try to stop them when they left the road.

The path they were following stayed in the forest for a long time, which made it easy to find a hiding place - usually a big tree, as awkward as they were to climb. Kerry sat just off the road in the shade of a bush or tree and dozed there.

The 'show' version of this clip could be used as part of a scene or a scene itself, and it describes them, their situation, and their habits pretty well. The 'tell' version of this clip, on the other hand, is not a scene in itself but is a lead in to a scene or perhaps a bridging structure between two scenes.

In this set of examples, the 'tell' version sets up their habits and allows you to start a 'show' scene when something starts happening, with one character on the road and two hiding in a tree on any day after they've developed the habit and before they leave the forest. The 'show' version allows you to continue the scene with the same character locations, but with a couple of differences: a lot more emphasis is placed on the argument and it seems to be a one-time thing not an ongoing thing as it is in the 'tell' version; and you are making an implicit promise that something will happen the day of the argument and not some other day. In this case, because such emphasis is placed on the argument that they will not be found, you just know that either they're going to be all found, or some of them are going to be found, possibly by the bandits they're arguing about and possibly by someone else. In the 'tell' version, the thing that happens might have nothing to do with the bandits, because their arguments are only mentioned in passing.

Both are good; you must decide which one works best for your story and what you are trying to do with that scene and the ones around it.

Anyhow, my point is, 'Show, don't tell' is advice that, like any advice, needs to be applied when it suits the situation and ignored when it doesn't. There are no hard and fast rules for choosing which 'writing rules' to apply and when - if there were, computers could write better stories than us, because if there's one thing computers do well it's follow instructions without applying judgement or experience.

'Show' generally is best for showing an important event or example from a series of similar events where the event itself is important, while 'tell' generally is best for setting up a situation, or describing a series of events where the overall view is important but no one individual event is worth making a whole scene for.