Whenever I teach students a slow piece, I always tell them to relax, and think of the melody line. It is important to always keep your chest open. Tension in the shoulders can affect the arms and wrist in a negative way.
Once you keep your chest open and relax, the sound created is just gorgeous. Take a look at this video of Yeol Eum Son's Final Recital at the 2009 Van Cliburn's Piano Competition. Her sound is just exquisite, that you can hear it over and over and it will still speak to you in different ways.
Look at her hands, her fingers are ready over the correct keys before she plays them. Her fingers and anticipating the shape and touch of the next chord. Listen to how "legato-sounding" her chords are.. it is definitely not percussive sounding. How many of us actually anticipate the notes or chords that come after?