Three of my students are essentially non-verbal, or at least they do not talk to me.  Questions are met with 1 word answers, if any.  In researching aphasia I discovered the technique of "phrase completion".  You say, or better sing, part of the verse  "Mary had a..." and the student just HAS to say "little lamb."  It is almost as if they cannot help themselves.

Of course, they are well set-up.  First I play and sing the target song or phrase.  Then I play piano and we toot on our kazoos (see blog on kazoos).  Then I play and sing MY part and the student toots HIS part.  After all that, the student SINGS his part in the song.  Thus far, ALL of my students have sung the words, in rhythm.  Usually after that the student will sing all the words to the song.  Finally they will play the song on the piano (maybe I will sing along or toot kazoo), and by then the song is learned and sung.

So far we have always gone on to another song, meaning the student will have talked/sung for half the lesson.

Tommy (TBI) was having nothing to do with the 6 white horses (She'll be comin' round the mountain).  He was an accomplished driver before the accident, and planned to go professional.  Today he cannot walk or talk, but he maintains and drives the tractor on the home property.  So, once he changed the words to "She'll be driving a 6 speed manual when she comes" he was singing and laughing.  Tommy's father has remarked that Tommy is talking so much more after just 3 lessons of this phrase completion activities in piano lessons.