Hello Morgan,
Morgan wrote:
In some screencasts I see mouse movements in perfectly straight lines or arcs, how is that done?
Firist off, I don’t know whether I have understood you correctly 
But, as regards “circles” around your mouse cursor, on Windows, with specialized software to screencast (e.g. Camtasia, BBFlash Back express, Camstudio etc) you can choose the size, the color, the transparence of this circle.
More precisely, this circle is only visible after you have stopped recording your desktop, when you have already saved your file. In order not to “distract” you while recording.
For instance, with Camtasia:
On Linux, I am not aware about any screencasting software which allows you to get this effect (circle around cursor).
Of course, you might create a new icon “cursor” ( maybe with Gimp ? ) with a circle around it, save it in the right folder (depending on your Linux Distro) and use during your desktop recording
On Kde, among the Desktop (Settings) options you can choose to show a different color for your right click or left click.
In the past, through VirtualBox, I had Windows 7 as Host and Ubuntu as Guest
As a consequence, I ran a Windows screencasting software on Windows (it was Camstudio back then) while working on Linux (Ubuntu, the guest).
Through Camstudio I had plenty of options (circle around cursor, different color for right-left click) and they were all available, later on, when I was editing my video recorded during the “Linux session” (through Vbox)