I would like to know if there is any difference between "devant" and "en face de". Are there any specific situations in which it would be unusual to use one or another?
Devant means "in front of", and so relates to location only, whereas en face de means "across from" (meaning there's a third thing to cross to get from one location to the other) or "facing" (meaning the two things being described have a literal or metaphorical "face" that look at each other).
Je suis devant vous dans la file [I'm in front of you in the line — can't use en face de here since people don't face each other in a line]
Je suis assis en face de Paul [I'm sitting across from Paul, eg at a dinner table].
Je suis assis devant Paul [I'm sitting ahead of Paul, eg in a theater]
Je travaille dans l'immeuble en face de la mairie [I work in the building that faces city hall, meaning the two buildings' "faces" are "looking at each other" across a street, square, river, or something like that]
Je vous attends dans le square qui est devant la mairie. [I'm waiting for you in the square in front of city hall; can only use en face de here if there is something that one needs to cross in order to get from city hall to the square.]