Spellsinger, maybe you refer to timing in one round - if so, it resolves when it's the players "mini-turn", i.e. when it's his or her initiative.
Usually, players declare what they want to do, they *then* roll initiative and resolve what they're doing one by one. This reflects that a round lasts 2 seconds, so in all that chaos things might be a little mixed up (like you want to help your comrade next to you, start mumbling the spell, but he's dashing off after some foe).
[To keep things even more realistic, I use combat cards, so players declare their options by choosing a card, unknown to the others]
So, to extend on what Rasyr referred to: If said comrade has left, you can still switch to an action that takes 0 rounds - casting an instant spell, e.g.