Partner's need more support after they miss a shot. I'll tell 'em, "That was close, you'll make it next time, keep trying it until you it"--keeps 'em from tightening up.
Also, if you're playing with a really good player or a pro, although they don't require it, they like to hear some kind words on occasion maybe after they've made an amazing shot so they don't feel taken for granted or used--an occasional look of awe is good too.
"...the human emotional system was not designed to endure the mental rigors of a tennis match." Dr. Allen Fox