Watch the 'Perry Mason' ATX Panel, Including Robert Downey Jr.'s Cameo! (VIDEO)

It's not just anyone (or any production company) that can go about creating a prequel to one of the most iconic TV lawyer characters in the medium's history but it took the strength of HBO and Team Downey (led by Susan Downey and Robert Downey, Jr.) to make it happen in the new Perry Mason, premiering Sunday, June 21 on the pay cabler.

If you've always wondered about the early days of the polished, self-assured legal eagle character (played by Raymond Burr in the 1957-1966 CBS drama), then you're in luck! In this new incarnation, Matthew Rhys (The Americans) portrays the character in a time in his life where Perry is still a low-grade detective and not yet a lawyer — and he's, well, kind of a mess. That is, until a case revolving around a murdered child comes his way and he has to step up and face his potential.

Matthew Rhys Shares 6 Things to Know About His 'Perry Mason' on HBOSee Also

Matthew Rhys Shares 6 Things to Know About His 'Perry Mason' on HBO

'This is not going to be [your] parents' 'Perry Mason,'' the star says of the prequel's surprising iteration of a beloved character.

To find out more about the development of the series, TV Insider's Jim Halterman moderated a virtual panel for the 2020 ATX Festival earlier this month. The panel took a deep dive into the shaping of the series with executive producer Susan Downey, as well as showrunners Rolin Jones and Ron Fitzgerald and executive producer/director Timothy Van Patton. Also on hand from the cast were Rhys, John Lithgow (The Crown), Tatiana Maslany (Orphan Black), Chris Chalk (Gotham), Juliet Rylance (The Knick), and Shea Whigham (Homecoming).

And, last but certainly not least, be sure to watch to the end of the panel where Robert Downey Jr. himself popped in to make an appearance!

Perry Mason, Series Premieres, Sunday, June 21, HBO

Watch other panels from this year's ATX Festival, on YouTube.