Iniciative is rolled when two or more sides decide to do something at the same time. You don't roll iniciative because a hobgoblin drew it's sword or because it is attacking you. You roll iniciative because you want to answer to his action. If he will try to attack me, I want to attack back! -- Two sides acting at the same time. Roll initiative to see who acts first.
If a PC fakes surrender and tries to charm person the guard: charm person has verbal and somanthic components, that means it is clear to the guard that the PC started to cast a spell. In the moment the guard notices the PC is casting a spell, he will go aggressive and will probably try to stop him. Roll initiative. Even if the PC wins initiative, the guard is already considered as 'fighting', because he already saw the PC tarting to cast a spell against him.
However, the player can try to cast the spell silently. He can murmur the words of power so the guard doesn't hear it and hide the hand gestures perhaps inside his mantle. If the PC can come with a convincing way to hide the casting and the DM agrees that it may be doable, the DM may ask the player to make a Dexterity (Sleight of Hands) check against the guard's passive Perception. If he succeeds, he gets a surprise round against the guard, where he casts the spell before the guard notices he is trying to do so. Note that other PCs can't take turns in this surprise round, since they didn't know that the wizard were going to try to charm the guard or exactly when, unless it was previously combined.