In most cases you will be expected to follow target calls made by your team, but by no means does that mean you cannot take the lead yourself when necessary.
Interested in playing MCH in PvP? Lace has created a video guide that explains some basic dos and don't dos to help beginner MCHs in PvP (regardless of Feast or Frontlines).
Lace Valeria highlights some Machinist action in Frontlines.