When should I cast my spell shield? Should I cast it before (for example) Nocturne casts his fear, or time it so that it's up when the fear procs?
You have to use Spell Shield before Nocturne uses Fear, otherwise the fear won't be blocked. The damage will also be negated if you use it before.
Spell shields will block the application of the tether but will not block the fear from a tether already present.