Thats odd, if it were doing it regardless of other fish interaction then id suspect parasites but only when coming in contact with another specific fish? How do you mean twitch?
Need alot more info can't help with that little bit of info.
Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia,nitrite,nitrate,and ph.
Need to rule water quality out first.
Any excess slime build up on the fish body.
Check the gills for being pale with excess mucas, or red and inflamed.