When moving inside the log buffer using n and p, then the revision buffer is automatically updates to show the commit at point. This can be controlled using the option
Until a little while ago that did not work well when holding down one of these keys, because that caused the revision buffer to be updated for each commit moved over in the log buffer. I just pushed a kludge for that, which works by delaying updating the revision buffer. You might want to customize the new option
magit-diff-auto-show-delay in order to fine-tune the delay.
Alternatively you can configure
magit-diff-auto-show to not automatically update the revision buffer. If you do so you can still display the commit at point in the revision buffer by pressing either RET or SPC. The former makes the revision buffer the current buffer, the latter does not.