Every panel is a little picture (or, in the case of a splash page, a not-so-little-picture). Every panel, however, does not need to be a complete picture.
Within the framework of a page, a comic, background and atmosphere are established, characters and their positions are shown, dialog happens. But it doesn't have to be in every panel; there's carry-over.
Detail is also an element of pacing. A more detailed panel invites the eye to stay longer. A panel without background detail suggests that you should move through it quickly. So both action shots and big dialog sections can actually benefit from reducing the detail and the extraneous information.
Each panel also needs to be harmonious on the page. The composition, the direction of thrust, the eye leading, the weight of shading; none of these can be chosen entirely by the needs of that specific piece of art, but need be modulated by the needs of the page around them.
That said, all the compositional elements and tricks are available.
Rule of Thirds: the focal point is rarely center. But this is a little more complex in sequential art, when there is activity taking place in the space between panels. The space of air in an individual panel is also a potential focus. To go back to the fist-swinging hero, the true focus of the panel is where his fist is going to be when that blow connects; the air between him and his target.
As a general rule, any panel should only have one prime focus. It can have multiple foci, but the others are subordinate. This gives a better balance in most cases.
Depth Planes: every panel is a window into space. It helps to shape that space by having elements that are foreground, mid-ground, and background. Obviously there may be more levels than that in a practical sense, but thinking in terms of three major, contrasting, planes makes for a clearer design. And when working within the constraints of inked borders and color fills, this division opens up possibilities such as making one of the planes a neutral silhouette.
Related to this, many of the masters recommend using three distinct values, and placing them so the largest contrast occurs between two of them; a dark foreground, a light mid-ground, a middle background, for instance.
Shot: every panel is a shot, and part of your overall shot scheme (including pushing in for emphasis, respecting the Line of Action, etc). Camera distance from subject and camera angle to subject nuances the emotional reaction to the subject, the more so when contrasted with the shots used in surrounding panels.
Perspective: although real perspective is complex, each panel should have a single overarching perspective scheme that frames the contents. A room is presented with specific, strong lines that help to define the deep space of the panel for the reader, even if there are elements in the room (chairs, say), that are angled to that scheme and required their own perspective plotting.
Perspective works hand in hand with the camera shot and the thrust of the panel; a long look down a foreboding corridor that leads the eye, points towards the goal, frames the focus, and is presented in strong 1-point-perspective, as one for-instance.
And it is worth repeating that many panels can be thought of as a "talking head" or an "insert" shot, where almost none of the above is used, or used minimally. But check out Wally Wood's "22 Panels that Work" for a masterful demonstration of multiple ways to break up the "talking head" shots and give interest to the conversation.
I have, or have read, over a dozen books on perspective, plus various tutorials. Above everything else I have ever seen, I recommend -- I strongly recommend -- "Vanishing Point" by Jason Cheeseman-Meyer.
It explains perspective. It explains it for a working artist (who doesn't have time to geometrically solve every loose card strewn on a poker table), and it gives things you simply don't see anywhere else. Curvilinear perspective, for instance. But even more than any of the above, it has stuff on how to figure out where to put the vanishing points. How to place them so you don't get that disturbing distortion. Tools that are simple, easy to understand and apply.
My preferred procedure is to rough out the page in thumbnail. I really should be making full thumbnails of each panel, or a cleaner rough, but right now I'm trying to draw faster, not neater. The next step is roughing in the panel plan, with lightly penciled contents, on the full sized sheet of paper.
Once I've seen the contents of a panel will fit and work, the lightly penciled freehand of the panel will form a guideline as to the perspective tools.
These fall into several rough classes. Some panels are essentially without perspective; either flat, or in planes. This doesn't mean the contents don't have individual perspective, but a talking head can be drafted as a head without having to construct horizon lines for the scene around it.
I always put a horizon line in, even if it is just a reminder of eye height in the scene.
Certain panels lend themselves to single-point perspective. I find this can be cleaner and make for a sharper focus. Even a complex city-scape may be nothing more than single-point.
Panels with boxes in them usually fall into 2-point. Here we get into traditional perspective; draw the horizon line at observer's eye height, plot two arbitrary points on it. I find -- again referring to Rule of Thirds -- it seems to work best to have one point in frame, and the other out of the frame. This latter point is usually realized by taping a ruler or other extension to the drawing board.
This is why for most perspective methods it works best to set up the vanishing points, and run reference lines to them (free-hand for a looser or organic scene, gridded equidistant for more technical/architectural scenes), and then put away those tools and proceed to draw the rest of the panel free-hand on those references.
Drama really comes with 3PP. In plotting these, the first advice I can give comes from Stan "The Man" himself; draw the first two points as you normally would, then drop the third "at some arbitrary distance" above or below the horizon line. A word here; bricks and buildings and so forth look decent only when they fall within the triangle (or between the two vanishing points of 2-point). Take a corner too close to a VP and it starts to look weird.
This is because, of course, artistic perspective is a cheat. It is an unreal tool that plots three-dimensional reality on to two-dimensional paper in a way that agrees with how our brains typically process the three-dimensional reality around us as captured through point sources (aka our pupils). The real world may have straight edges but we don't actually see them that way as projected through our eye, any more than they photograph that way (no matter which particular scheme the camera lens maker has used to try to distort the projection to better suit our preconceived perceptions).
And, yes, there are people who plot out parquet floors in exquisite detail. If you want to draw like M.C. Escher, then go right ahead. I want to draw a comic book before I turn grey. So I draw rough guidelines and fake the rest.
Yes, I will rule major lines back to the vanishing points when it seems required. But for the most part, the most elaborate I get is to construct a grid. Here's simple geometry; make tick-marks an equal distance apart and draw lines from them to the vanishing point. Draw a diagonal line through them. Now lines drawn perpendicular to the edge you measured will be equidistant in perspective -- as long as you make sure to draw them where the diagonal intersects the first lines.
Ah, but how deep is the shape? For that, unfortunately, one needs to plot a couple additional points. These are basically the lens width; putting these closer together flattens the image like a telephoto, putting them further apart makes the depth stretch Vertigo-style. (That's in one-point -- two-point is slightly different).
As I said above, in my own particular art style many panels don't need elaborate perspective. I can fake up a figure with nothing but a horizon line, and maybe some arbitrary scribbles on the figure itself to remind myself where the vanishing points are.
The last thing I want to mention is how helpful it was to load up a second mechanical pencil with red leads. This lets me put down perspective guidelines that won't confuse me later during development or inking. Even if it is just one horizon line...or a dot helpfully labeled "V.P."