Dialogue should help the reader get an understanding of who the character is, while also moving the story forward. Avoid being "on the nose"  or having character's say exactly what is on their mind as the subtext, or what is left unsaid, ads tension and emotional resonance to a scene.

If a character is interrupted during a speech, it should end with an M-dash (space followed by two hyphens). It is not necessary to start the character who is interrupting the speech with an M-dash. 

Like this:

       SHARON
Is everythi --

       TOM
It's here!

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If a character is reading a block of text out loud, this should be established with a parenthetical (reading) and the text enclosed in quotation marks.

Like this:

             SHARON
      (reading)
"Dear Thomas, The admission
committee has completed its review
of your application."

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If a character's speech trails off, it should end with an ellipsis (...). If a second character finishes that sentence, their speech should begin with an ellipsis.

Like this:

              BRENT
We even finish each others...

Brent is distracted by a fly.

              KYLE
...Sandwiches.