Freelancers get called when there’s a problem; a pitch, or an assignment that can’t be cracked. Realize that some people on staff at the advertising agency may think you’re there to swipe opportunities that otherwise might go to them. Tread gently.
Try to be sure your success doesn’t come at the expense of someone else. At least be aware that that’s the way some people might view your efforts. Make it clear that you have no ambitions except the success of the thing you were hired to work on. You’re not angling for glory or a full-time job.
Fix it, then vanish like the wind.