apt = suitable?

I understand both apt and suitable are less common word to refer to right, or appropriate. Other than meaning is there some restrictions in their usage in which one can't be used in a place the other can be used? Or are they interchangeable?


apt
suitable or appropriate in the circumstances
"a particularly apt description/name/comment"
http://www.oxfordlearnersdictionaries.com/definition/english/apt?q=apt

opportune adjective (of a time) suitable for doing a particular thing, so that it is likely to be successful
synonym favourable
The offer could not have come at a more opportune moment.
Chapman decided the time was opportune for talking business.
http://www.oxfordlearnersdictionaries.com/definition/english/opportune?q=opportune

They're not interchangeable, no.

I would say the essential difference is that "apt" means it fits the context or circumstances, whereas "opportune" conveys the sense of it's going to give a successful result.

So opportune conveys the sense that it's suitable for the future result, whereas apt highlights on it goes well with the circumstance in the present. Thank you, DonnyB.