To avoid flat dialogue scenes, learn to triangulate the characters’ interaction with a lower-order goal.
Is it better to format character thoughts with quote marks or italics? I say neither. No special formatting is necessary to signify character thinking. You just need some solid narration.
Should you use synonyms for “said”? Some writing advice claims “said is dead”; others look disparagingly upon “saidisms,” the fancier cousins of “said.” Here’s the rule of thumb.