When you start a new main Mission (such as Mission C7A) you chose which of the heroes on your team will go first and the enemy goes after that.
HOWEVER … often (always?) when you complete a mission, you turn the page in the Campaign book and there is a Mission Update. If this update is a continuation of the current mission, then the Enemy will go First… ALWAYS.
The final game will have a reminder about this at the top of every applicable Mission Update!
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