What is the difference in meaning between repress and quell?
Are they interchangeable?
I would say that repress has more negative connotations than quell. One might "brutally repress an uprising", for example: it suggests that force (or similar severe methods) are used. "Quell" is more suggestive of gentle reassurance or reasoned argument: "the priest quelled their fears".
Linked to that, there's an implication that something "repressed" doesn't really go away; it's just no longer in the open. "Quell" suggests that issues have been smoothed over.
Repress: "to check, restrain," ... "hold back, curb," figuratively "check, confine, restrain, refrain," ...- "back" (see re-) + premere "to push" A terms later associated with psychological repression, and arresting a rebellion -http://www.etymonline.com/
Whereas, quell was from the get go "to kill, murder, execute".
put an end to (a rebellion or other disorder), typically by the use of force.
/subdue (someone or something) by force.