Here's quick tip to get nvidia working properly with bumblebee 3.2:
if you get this message:
$optirun -vv --debug glxspheres
[ERROR]Cannot access secondary GPU - error: [XORG] (EE) Failed to load module "nouveau" (module does not exist, 0)
[ERROR]Aborting because fallback start is disabled.
Assuming that you blacklisted nouveau, and want the nvidia driver instead, simply delete busid.conf under
and everything will be back to normal
Would be good to add this to the troubleshooting section here (again this problem seems to occur with bumblebee version 3.2):