In the "if t'were a snake itta bit ya" category the Options Bar appears often. The question asked, "Can I copy something multiple times?"...the answer, "the Options Bar".
Another, "Can I copy something when I rotate it?"...the answer...you get the idea.
The subject of this post is, "Where is the fillet command?" The answer..."the Options Bar, sort of". Trouble is that most everyone seems to think that the fillet command is an editing task. All those years in other software told em so!
Along comes Revit and they seem to view it as a creation tool. A fillet is creating a third element, a line or wall arc where nothing was before except two lines or walls meeting at a corner or maybe not even meeting. With this in mind the process to fillet between two walls or lines is a creation event and the command is hiding within the commands for both, wall or lines.
When you are sketching a wall/line you have to take a closer look at the options bar because there is a little innocent check box next to Fillet and a place to enter a radius value. Check it, enter a value and when you sketch you'll get an arc between the segments you draw. Like this box:
Now what if you have a box already and you really meant to have radius corners? Start the wall/line command again and this time take another look at the Options Bar. Click the little pull down arrow next to the sketch tools and look at the list offered. Choose the Fillet Arc option.
Now you can select the two walls/lines you want to fillet between and graphically decide where the arc should land or specify a radius on the Options Bar first.
When in doubt, take a good look at the Options Bar!