Sometimes interior design and architecture is a puzzle. You're given certain pieces - usually requirements from the client, maybe some measurements of the space to work with - and you need to provide a solution to the problem (puzzle) you're given. Within the context of blocks (working with forms), we can look at it like a tangram puzzle. In the client's case there may not be just one right answer, but it's our job to figure those answers out and provide the best fit.
Sometimes even using the previously- mentioned tools (dimensional transformations, subtractive forms, additive forms) isn't quite enough to provide that best fit. We have one last tool in the box: formal collisions of geometry. The name itself is pretty suggestive of what it is, but to clarify, it is using at least two forms (as they are, or rotated) and combining them together so that they become a new form. (Remember the movie The Fly with Jeff Goldblum? That was a formal collision, indeed.) Any forms can be used, and a look at a wide variety of floor plans will show that it's used quite a lot when a certain effect is desired, or the space one is working with is challenging.
Both edges and corners are important, but when considering interior design, how you address corners in particular is important for defining (or not) the space. The key is that the angle between your planes - the corner - must be such that the corner does not simply resemble a wall. In other words, an extremely large angle won't feel like a corner. Most corners are formed with right angles (90 degrees), but as a general rule of thumb, one could realistically work with angles anywhere from 60 to 120 degrees. (Cases could be made for smaller or wider angles than these, but it would take a truly talented designer to do so.)
Angle alone isn't the only concern - what shape could the corner take? Any, really, and sometimes there isn't anything there at all. Sometimes a lack of definition provides definition by its very absence. The corner can fold inward or jut outward, can be smooth or highly articulated, and so on. Consistency is key.