Julian and Joaquin Castro Weigh In on Hillary Clinton, Donald Trump

— -- In an interview with ABC News' David Muir, U.S. secretary of Housing and Urban Development Julian Castro said Hillary Clinton just needs to "be herself" when she takes to the stage this evening to address the Democratic National Convention in Philadelphia and formally accept the party's nomination for president.

"She's been in the public eye now for almost 25 years," said Castro, "and so part of the challenge is, that people are not suddenly going to think of you in a totally different way...I think if folks listen to her, that they will see, as [former] President Clinton said yesterday, who the real person is and instead of the caricature that's been created by the Republicans."

Castro, who spoke with Muir alongside his twin brother, Democratic Rep. Joaquin Castro, at the convention Wednesday, was vetted early in July by the campaign for the vice presidential spot. Sources said that he and Massachusetts Sen. Elizabeth Warren had met with Clinton at her Washington home. Castro would have been the first Hispanic candidate on a presidential ticket.

On July 22, Virginia Sen. Tim Kaine was announced as Clinton's vice president pick. Julian Castro told Muir that not being selected had been a bit of a letdown.

"I'd be lying to you if I said there wasn't a little bit of a disappointment," he said, "but I also know that I'm 41 years old and that God willing, I'm going to have a lot of years left and I know that I got into public service because I felt very blessed with opportunity...Whether I'm in the public sector or the private sector, I'm going to find ways to create more opportunities."

Julian Castro said further that Clinton had made an "excellent choice" with Kaine.

"He is well-prepared. He has been a governor, a senator. He understands the world," Castro said. "He's a great choice for VP."

Rep. Castro, who represents San Antonio in Congress and was scheduled to speak today at the convention, shared with Muir some of what he planned to say.

"I'm going to talk about the reason that people come to this country and that is opportunity and that Hillary Clinton will help create that opportunity and I think Donald Trump will tear it down," he said.

Joaquin Castro said he would not be shying away from using the Republican presidential candidate's name in his speech.

"I'm going to call him out, yeah, directly," he said. "There is a lot of frustration and anxiety in the country but as an elected official and as a politician, you can direct that energy in different ways and he's directed it in a very divisive and negative way and really played on people's fears and resentments."

"This election truly is about a choice we're making between a candidate in Hillary Clinton that has a broad inclusive view of America and in Donald trump someone that has a darker sort of exclusive view of America," Julian Castro said.