"It made me alive to the fact that the most important thing sometimes is what isn't said – to prepare for moments of revelation that can be read entirely on actors' faces without dialogue." -Robert Towne
Mr. Towne (Chinatown) brings up a valid point; one that can be forgotten quite a bit. Most of the time less is more. Heavy exposition can bog down a screenplay like an elephant strapped to a life raft.
When going back and editing dialogue, make sure it sounds real (reading aloud helps here) and that it's concise. When a character over explains the situation he or she is in, it seeps through the script, clouds pacing and can turn off a reader.
These questions will help you clean up that excessive dialogue:
1. Do I need that sentence? Or that word?
2. Do these blocks of dialogue between characters fall in line with previous [edited] dialogue blocks?
3. (After the first run of editing) Does the page of dialogue you just read develop your character(s) further? Either for the positive or negative?