When Revit got its start, round about tens years back, one of the intentional concepts it hoped to bring back to us is sketching. Again not necessarily the napkin-sketch-look but at least the idea that we can draw something without being too hung up (at first) with accuracy, exact dimensions or precise wall types.
You can draw walls that loosely define rooms/volumes/spaces and shapes. After messing around with the general form and arrangement you can force more rigor or rules on the concept. You know, like a grid layout, masonry brick/block coursing/modules or assumptions about minimum clearance or sizes. I still encounter users now and then that start out by drawing "lines" to decide where walls should go. No, No...missing something big here...
The key thing to appreciate is that you can just sketch first and refine next/later.
I realized shortly after starting to use Revit that I was conditioned to try to draw really accurately FIRST with AutoCAD and Microstation because it was a pain to adjust things. Don't get me wrong, some change is painful even in Revit. Completely throwing out a concept will inevitably require a lot of rework. Starting with a rectangular building and switching to an oval probably isn't going to just update nicely, completely.
With Revit routine adjustments to clean up a concept IS much more friendly. Some of the tools that let you refine your work are temporary dimensions, permanent dimensions, levels, grids, reference planes and parameters, editing tools like move, align, rotate, the Type Selector and a little smarts and intuition thrown in.
To support the somewhat arbitrary state of ideas that are not fully formed yet you can also work with placeholders for walls, doors and windows etc. This means that you CAN draw a nominal wall that is 4 inches (100mm) or 6 inches (150mm) or even 8 inches (200mm). We don't need anything bigger than that do we?? 8-)
As the design get resolved you can replace these nominal walls with ones that represent framing sizes, gypsum wall board choices and even finishes if you choose to. One excellent suggestion that I think was first offered by Phil Read in a session at Autodesk University several years ago is to use specific materials for all generic "placeholder" elements. He suggested a common color that used some sort of transparency setting so they were clearly different from "real/resolved" things. Since we tend to spend a fair bit of time looking at the model in 3D views with Revit these unresolved elements stand out in the crowd.
To further explain this stuff I recorded a short video embedded here and posted at OpEd Videos and You Tube.