I view the pacing of a film as a fine art that's part technical and part feeling and when I'm the one creating the pace, it's sometimes tough to know when it's on spot. One day I might be sure I've got it, the next I could have a slightly (or completely) different interpretation. Finding the flow comes easier with some jobs than others, but nevertheless I think it's one of the most important talents to develop and refine as an editor.
The narrative progression or mood of something is defined by the footage the filmmaker has shot and how well an editor puts the pieces of it together. In other words, what and how an editor uses those hours of material to build conflict and resolution in a dramatic and/or comedic, engaging way while being true to the story is key.
Sometimes spending a lot of time with footage can provide insight or it can do the opposite and cause a loss of perspective. A good thing about sitting at a computer forever, reviewing the material repeatedly is that it informs what possibilities lie in wait. Getting to know footage so well can also lead to an editor's own one sided relationship with it. This can be an asset or a liability.
Having unique familiarity with characters that don't know you can sometimes throw off the flow. I think it's important to to step back. For instance, getting intensely involved in material can allow for interesting discoveries that sometimes create the magic of the pace and/or plot, but an editor can also get so familiar with the footage, they think information is in their cut that actually isn't. Even worse is when an editor thinks something or someone is more interesting than they actually are. That's ugly and can most definitely throw the pacing out of whack. As I gain experience, however, I'm slowly finding ways to avoid some of those traps.
I like to keep a list of objectives for each job and continually return to that list so I don't loose sight of the purpose. Then I can ask myself if each scene serves the greater purpose or message of the film or video. Do the characters play to that purpose in an interesting way? When I ask friends, filmmakers and other editors to screen work, I request blunt feedback. I want to know when someone is bored or confused, when something drags or flies by so fast that an important emotional moment or narrative point gets lost. If there are common comments from several people that's usually a pretty strong indication there are areas to work on. I also like to examine how much information I'm throwing out, how clear and simplified it is. Is there a little breathing room (or too much) between thoughts. There's a long list to look at and it can easily get overwhelming.
If I stick with a short list of priority objectives or questions for myself, I can usually stay on track though. I try to ask if the pace builds a momentum and if so, does it build at the right times in the story? Do all the parts/scenes that make up the whole compliment each other, or do they at least set against each other effectively. It sounds simplistic, kind of Editing 101, but it's easy to loose sight of those elements.
Finding the right pace is, I believe a talent and a skill acquired over time with practice. Even with all that on your side, an editor must step back and remove themselves from something to try and see the assembly through fresh eyes. That's tough and so far, I can only do it to a certain extent. I need a lot of other people for feedback and make revisions off a combination of my own sensibilities and the various gracious screener consensus from there.
An editor can find a different pace for every individual project and he/she can also find a sort of signature pace. Some editors cut with a slower or faster style than others. Pace has trends too and if you ask me, trends in pacing are a bit of a waste. They come and go, but good pacing is good pacing.
Sometimes an editor can't control the pacing and I'm not referring to finding the organic flow and the film grows into itself. I mean sometimes the producer/director/client prefers a certain pace and an editor often must conform to their wishes, for better or for worse. Hopefully most of the time it's for the better.
The pace of things matters regardless of the project and it's often personal taste that defines what the "right" flow is, which makes finding it tricky. Like any visual medium, there is a rhythm to it but if it's off the mark, all your hard cutting is for not because our final judges are the audience. If your work falls flat, there won't be anyone watching.