SPEAK and TALK are almost synonymous and are generally interchangeable except in fixed expressions.
Remember that the verb speak tends to be used in more formal or one-sided situations. Unfortunately, there is no simple or easy rule to follow. But there are a number of fixed expressions for both verbs.
Always use SPEAK in these fixed expressions:
- Speak a language—(e.g. French, English, etc.)
- Speak for someone—voice what another person thinks
- Speak now or forever hold your peace—this is your last chance to say something
- Speak your mind—say what you really think
- Speak out (about something)—voice your opinion (on a subject)
- Speak up—raise your voice
- Speak up for someone—voice support for a person
- Speak volumes—convey lots of information
- Speak well of someone—say only positive things about that person
- this Speak for itself—requires no explanation
Always use TALK in these fixed expressions:
- Money talks—money can get things done
- Talk away—talk a great deal
- Talk back—answer rudely or disrespectfully
- Talk down to someone—be condescending towards a person
- Talk shop—talk about your job, using jargon
- Talk the talk—say things and make promises to please others
- Talk things over—discuss something
- Talk tough—speak in a brash or threatening manner
- Talk through your hat (or talk nonsense)—say things that make no sense
- Talk turkey—speak frankly
