Dream job, you'll be fine. McDonald's let's you see the whole spectrum of people I'm sure.
just think of a few examples you can give them... e.g. a rowdy patron, when you went above and beyond for a customer, when did you have a conflict with a colleague or customer and how did you deal with it... etc etc. I'm sure you'll be able to think of lots.
just keep in mind when you give your example "START Situation Task Action Result. What was happening? What did you need to achieve? What action did you take? What was the result?
don't be afraid to give a negative example where something didn't work out (They will likely ask one like this anyway) just remember to focus on what you learnt and how you would approach the situation differently next time.
good luck!