How to Write realistic Dialogue

Dialogue should be motivated by the circumstances in the scene and should be consistent with the character development already established.

The Top-ten Worst Problems and solutions in writing dialogue:
1) Too head-on or too on the nose. This dialogue is too literal and embarrassingly obvious. It sounds contrived.
2) Too choppy. This dialogue is staccato. Filled with one liners. A word or two.
3) Too repetitious. Dialogue becomes repetitious when a character repeats himself or herself in a number of different ways. the character offers redundant information or repetitive phrases.
4) Too long. Dialogue that is too long reads like an editorial speech or a philosophical diatribe. it creates static action in the script and often includes related problems of redundancy and prechiness.
5) Too similar. Sometimes characters sound the same. Their dialogue patterns are indistinguishable from eachother. Once that happens, the character's individuality has been lost.
6) Too stilted. This dialogue sounds as if it came from a history book, a poem, a newspaper, a grammer text, but not from a person.
7) Too preachy. This problem is related to being "head on," "redundant," "too long," and "too stilted." The character tends to sound very formal and espouses thematic ideas or philosophical notions. He or she becomes an ideological mouthpiece for the writer, rather than a dimensional being.
8) Too introspective. The problem deals specifically with the character who is alone and speaks out loud.
9) Too inconsistent. This means that a character is saying something that doesn't fit the personality already created. The dialogue is incongruous with character. In some cases, that inconsistency results from a lack of proper transitions in the scenes.
10) Too unbelievable. This is a catch-all category that implies that a character doesn't sound real for any number of reasons. You can test the credibility of dialogue by speaking it loud to judge whether or not it rings true. it should sound like a real person responding to the immediate circumstances we've just seen.