Writing harmony parts for duet?

Question:

Writing harmony parts for duet?

I'm trying to write a harmony part for a short piece of music, that will be sung by one soprano and one tenor. I want to achieve that classic intimate sound that you often hear in a duet of this type. You know, where it sounds intertwined, like their singing to each other (take Summer Lovin from Grease, for example). Any idea which intervals or principles to use to accomplish this sound? Thanks!

Best Answer - Chosen by Asker: Generally, for duets you want to have intervals of thirds (like C to E) or sixths (like C to A). As the melody line moves, you can follow it a third above (keeping in the key) or sixth above (this is a little trickier because the harmonies imply a different key). If you simply follow the melody, it's called a parallel harmony, and can get kind of boring, so use passing notes to move from thirds to sixths (using fourths, fifths and seconds.) for example, in the key of C, a melody might go: C-D-E-D-C, and a parallel harmony would go E-F-G-F-E. But a more interesting harmony might be E-F-C-B-E or E-F-A-B-C (unison at the end) (by the way, notes are only part of what makes good music. Remember dynamics, Phrasing and range, too)