Whether I'm playing with Lori or with Lynnette Suzanne of Tea for Two, I really enjoy performing in a vocal/guitar duo. It's a big challenge to "be the band." You have to tastefully support the singer while covering harmonies, bass lines, and lead lines, and you also have to play some solos without the benefit of another instrument, which is particularly challenging for a guitarist. The singer has challenges, too. She doesn't have the benefit of a full band backing her up. She has to generate a lot of excitement and interest, but she has to be able to do so nimbly so she doesn't overpower the guitarist.
Another big challenge in accompanying singers is being able to play songs in a variety of keys. If I only had to worry about myself, I would just have to learn a song in the standard key, plus "my key" if I'm going to sing it. When I play for Lynnette, I usually have to transpose a song a 4th or 5th (4 or 5 notes) away from my key. Lori's voice is a little lower than Lynnette's, so if I'm accompanying Lori, I usually have to transpose a 3rd lower than Lynnette's key.
As Lori finds her voice, she has had me transpose songs into several different keys. I give her a hard time about it, but I enjoy the challenge of learning songs in different keys. I often have to completely rework an intro and find different chord voicings. Different chord voicings will often suggest different lead lines. In our rehearsals, when we're finding Lori's key for a song, I may have to transpose the song two or three different times, which is a great skill to develop. My transposition skills are improving. I'm not quite ready to transpose on the fly in a public setting, but I'll get there.
To me, the epitome of a great vocal/guitar jazz duo is Joe Pass and Ella Fitzgerald. They were both world class musicians, and when they performed together, it was magic. I aspire to play like Joe Pass when I accompany a singer. I'm not even close to his level, but I'll be doing my best Joe Pass imitation at Java Monkey tomorrow.