Can I complete quest components without signing up for them?

Question

I was flying around Metropolis and saw a bunch of people fighting some Brainiac Troops. Based on what was going on it looked like it was part of a quest (people were being stored in orbs). I stayed for a little bit a killed a bunch of troops.

Many of the quests I've done so far include things like "Kill 20 Venom Agents." Later if I accept this mission, will I receive a credit for the units I've already killed?

I know in game series' like Diablo and Dungeon Siege you can complete a quest without having signed up for it, as long as you finish the requirements, you just receive the reward.

Answer 1

Overall, no.

In quests you have to explicitly accept, the "kill counter" doesn't start ticking until and unless you've accepted the quest, and the fifty VENOM agents you already incapacitated won't count toward the fifteen that your contact wants you to take out.

On the other hand, "automatic" quests are added to your quest list the moment you pass through the area they're in so your kills start counting right away.