Talking with conviction: Deborah Meadon's guide

Deborah Meaden says that the ability to spot somebody who can deliver with conviction is crucial, both in her day job as an entrepreneur and her TV role in Dragons' Den.
"Find the thing that you are passionate about," Deborah told us: "Automatically you'll deliver it with conviction. You believe, they'll believe it".
Even the best speakers shortchange their audience if they're not delivering with conviction.
Deborah's seen great speakers disappointingly speaking by numbers: "They were speaking very well... but I didn't come away thinking- 'I'm going to do something now'".
Perhaps that's the hallmark of a truly believable speech?