Many, but not all, interactions are known and assessed in advance for balance, interest, etc.

Sometimes, we build around them intentionally. The point of a summoner spell, item, or ally’s kit is to change something about how your champ plays. This often involves changing the nature of a spell. Indeed, these changes are almost inherently more interesting than simply amplifying a spell (as AP tends to do).

Sometimes, we spot them emergently in internal testing. Most Rioters are experienced League players—we’ve been playing for years. We tend to understand the probable ways to break a kit that have been seen before. Even when unplanned, if spotted, the team can backtrack and re-design the interacting spells to be balanced/fun/compelling/etc.

Sometimes, you all surprise us. This is particularly true of champion interactions, since there are so many (there are currently 45 billion unique five-champ compositions available). In this case, we often have to go back and evaluate how we want the interaction to work, but we approach the situation with a strong bias toward supporting the strategies players have discovered.

Once identified, we ask whether we believe we can support the interaction. If we can’t, we need to either disable or modify it. This is a cost/benefit analysis: Is the cost of combining or investing in the two elements of the interaction sufficient to make the combo balanced?

For example, the Bodyslam+Flash combo yields a dramatic increase in reliability. It’s honestly a degenerate interaction. However, it involves Flash, so the massive cooldown/power level makes the opportunity cost of using the combo very high to compensate for its strength.

Fiddle’s Crowstorm+Zhonya’s combo gives a dramatic reduction in risk, but actually reduces the reliability of Crowstorm, since by Hourglass-ing, Fiddle roots himself. Typically, these are the most interesting and long-term healthy interactions—where the costs and benefits are actually internal to the spell being affected.

A recent example of this actually comes from Zoe. We enjoy supporting Flash + cast time spells (e.g., Alistar Pulverize+Flash/Ahri Charm+Flash) to increase reliability. We thought it would be fine to support it on Zoe’s E, Sleepy Trouble Bubble. However, as she has access to more than six times the Flashes of a normal champ, the typical rationale for that support (the long CD and opportunity cost of burning Flash to increase the reliability of an offensive action) became inapposite. Unfortunately, I came to this conclusion very late in her development cycle, which left few elegant solutions. In the end, I decided to lock out Zoe’s Flash during E’s cast in order to allow her to be about more than setting up E+Flash plays. After all, why would you cast a risky E over a wall when you could just guarantee the hit?

Perhaps we will revisit that decision in the future, either by providing a less artificial blocker to the interaction or by re-assessing whether Zoe’s ability to frequently set up E’s with Flash is actually unhealthy. Such is the benefit of making a continuously updated game.