Fugue Playing

Because its difficulties arise primarily from the interaction of lines (and not the inherent difficulty of any line by itself), contrapuntal keyboard music can put a uniquely dense and heterogeneous set of demands on the performer.

A vivid example

The final stretto of Fugue 5 in D Major from Book II of J. S. Bach's Well-Tempered Clavier (standard and bar-graph scores shown below) provides an especially vivid example of such demands.  Though the passage requires little technical brilliance in the usual sense (it chugs along at the leisurely pace of two eighth notes per second, there are no difficult arpeggios, leaps, ornaments or other obstacles; the only technically awkward move involves contracting the right hand somewhat more than is comfortable in order to place the fifth/pinkie finger in the position that would be most easily occupied by the third finger at eighth note 16 in the diagram), changes in what each of the four voices is doing are both varied and rapid happening on almost every eighth note.

In homophonic keyboard music, the activity of each hand can be chunked fairly easily, and the number of pending planning events (upcoming changes in what you're doing that you have to prepare yourself for) is typically one or two. In contrast, the number of pending planning events in this passage ranges from two to four, with one change happening almost every eighth note (n.b. in the bar-graph score, the size and color of notes highlight differences in their duration and articulation):

Here's what you need to do in preparation for each of the twenty eighth-note beats in this passage:

List of planning events

"Nothing new" means that you keep doing something you began doing previously, such as ...

    ... hold a note you previously played with the intention of holding it.
    ... repeat a note once you've started repeating it.
    ... keeping a scale going once you've started it.

(These are likely to be things that you can do without attention before attempting to play a fugue.)

Okay, I can do all that; what's next?

When you can reliably perform this sequence of changes of attention at some tempo, you can start to add other things to the mix.  As you practice, more and more of the original planning events will happen automatically, leaving you with some attention left over to direct to new ones.  New planning events added to the mix will tend to displace the original ones, though, so you'll need to learn some new combinations.

One of the first things to add is the ability to attend to any of the lines (soprano, alto, tenor, bass) continuously.  If you're not sure whether you're attending to a given line, try singing along with it (doesn't have to be in the same octave as what you're playing).  Although this involves no changes in what you're doing with your hands, it can be the hardest thing to add.  It's essential, though; without it, you can't accurately judge the effect of subsequent changes on the expression and continuity of the individual lines.

If you can play the passage reliably regardless of which voice you're directing your attention to, you're in a position to make meaningful interpretive changes, such as changes to articulation, dynamic balance between voices, expressive rhythmic variation, bringing out important parts of the theme, etc.

What if I can only think about one note at a time ... ?

A useful technique for learning how to bring out the independence of the voices is to stagger the start times of the notes so that at each notated simultaneity, you play the notes in order from least to most important. Deciding the relative importance of the notes is an interpretive choice; in this passage, I've ranked