"言う" and "話す”

”言う”　と　”話す”　には違いがありますか？
What is the difference between ”言う” and ”話す” ?

”言う” say
"話す" say, communicate, talk

(a) スーザンはスティーブと彼の事業の戦略について話しました。
Susan talked with Steve about the strategy of his business.

The sentence above represents a bilateral communication. So "話す" is suitable. "言う" is not.

(b) スーザンはスティーブにその戦略ではうまく行かないだろうと言いました／話しました。
Susan said to Steve that the strategy will not work well.

The sentence above represents a Susan's opinion. This case, both of "言う" and "話す” can be used.