Difference between collocation and fixed expression?

Dear Teachers and native speakers,

Could you explain what’s the difference between a ‘collocation’ and a ‘fixed expression’ ? I’d really like to help my co-learner Shafqat to understand why you should say ‘interested in’ or ‘keen on’ or ‘mad about’ or many others like that. I know that all these words’ clusters are needed to remember, but I’ve forgotten at the moment how I should call them.
Help me (us), please.

Best wishes :slight_smile:

The ones you have asked about would all be called collocations.

Fixed expressions are any sort of unchanging word combinations, including collocations, idioms and proverbs.