When I recruited I always put 'has to display initiative' in the advert and the job specification. When I then asked at interview how a candidate had done this I'd say 99 from 100 couldn't answer. I don't know what line of work you are in but here are a few examples that might help you.
I devised a carry-forward system so that I could follow up on any queries within 24 hours.
I designed a quicker paperwork flow which saved time.
Obviously you need to have shown initiative to be able to discuss it . So what did you do in previous jobs that went beyond the normal job? Those things will be where your initiative comes from.