Always hear the music before you play. In this clip of Glenn Gould practicing Bach's Partita no. 2, we can hear how he sings as he plays. Later in the clip he runs into a difficulty. He stops, walks away from the piano, sings his vision for the phrase, then returns and plays it while singing.
It is only with instrumental music that musicians have the unfortunate luxury of disconnecting from their playing. A singer would never simply not think of the note they are singing. They must imagine how it should sound. If they did not have the note in their vision, the note would not sound. With a musical instrument, however, it is often easy to think of other things, such as the physical aspect of our playing, and become distracted from the music itself. When we focus on the note-it's pitch, tone, color, we are able to implement the physical aspects more organically and purposefully.