If I understand Karl's situation correctly, he is holding the first note longer than the rest of them in a group.
I think this way is especially nice - in a trill and in certain other ornaments it sounds very natural to hold the first note a little longer and then speed up. Otherwise, if you play all notes evenly, it sounds a bit too automatic. And don't forget to start a trill from the upper note on the beat (not before the beat).
This tradition of starting the trills from the upper note comes from the French music and since Bach was heavily influenced by the French tradition in various ways, he himself advocated ornaments to be performed in the French way (however, certain pieces which were written after Italian fashion might have trills which could be started on the main note).
When Bach wrote the Klavierbuchlein for his nine-year-old son Wilhelm Friedemann, he included this table of ornaments (fashioned after the table by Jean-Henri d'Anglebert) which you might find useful to consult with. Note that the trill is shown to be executed rhythmically in 32 notes but in reality they would make the first note a little longer (like we do when we want to emphasize the downbeat of the measure in early music)..
What about you? How do you play trills and other ornaments in Bach's music?