To leverage the situation you need to figure out:What’s their time frame for a hiring decision? How long has the position being open? What kind of deal can you get with their competitor? Have then made any offers yet?
Lastly, do your homework to understand what’s in the deal for the interviewer. Can they afford to walk away?
Like Donald Trump said: "The Worst thing you can possibly do in a deal is seem desperate to make it. That makes the other guy smell blood, and then you’re dead.The best thing you can do is deal from strength, and leverage is the biggest strength you can have. Leverage is having something the other guy wants. Or better yet, needs. Or best of all, simply can’t do without."
—DONALD J. TRUMP, TRUMP: THE ART OF THE DEAL