A writer either finds writing dialogue easy or a real pain...something they sweat out.
Either way, if you don't tell your reader how your character is acting while speaking, you don't make it easy for them to truly start the movie in their mind as you see it in yours.
This goes beyond dialogue tags that indicate who is saying what. It also shows what they might be doing.
Let's face it, when you're talking on the phone, are you perfectly still, not moving at all? Not responding in facial expression to what the person at the other -- and unseen -- end of the conversation is doing?
When you are speaking, this is your point of view (POV), isn't it? But to a friend or family member who happens to walk into the room, this is what they see you doing.
Readers get to HEAR both sides of the conversation but they only get to see what you -- or your POV character -- is doing.
NOTE: If two people of the same gender are conversing, make it easy on your reader by indicating which is which by keeping close tabs on the "she" and "he" and "him" and "her" usage. Name them or find another designation that easily identifies them for the reader. Things like "the older man", "the younger woman", "the giant", "the child", "the manager", "her sister", "his father".
Not only do these things clarify things for your reader, they add those all important words for your daily tally for the NANO competition!