The star answered a ton of questions and was his usual funny self, so here's a rundown of our favorite things we learned from his AMA.
1. "Sherlock 3" Is In Development
Downey updated fans about the status of the next "Sherlock" movie and gave a tease about another character he'd love to revive.
"Yes, we have a 'Sherlock 3' in development. We want it to be the best of the series, so that's a pretty tall order. Also, Perry Mason is a property I'm looking forward to developing further, going back to the 1931 book series...similarly to what we did with the first 2 Sherlocks, mining the original material for things that are 'new.'"
2. The Source Of Iron Man's Power Is Named Pedro
Finally, we know the source of Downey's super-powered facial hair.
"It started with the first Iron Man: a guy named Pedro from Shave of Beverly Hills. I mean, this guy's an artist. He's like the Edward Scissorhands of Mustaches and Beards. And then, once you set a standard, it's all about upkeep and modulating it. It tends to be a lot of work, but I'm glad you appreciate it."
3. Who He'd Play On "Drunk History"
Downey called this an "excellent question," and I have to agree.
"Probably our hemp-headed forefathers. I guess they'd be called Founding Fathers. I'm pretty sure Jefferson grew a bunch of weed..."
4. What He Thinks Of Benedict Cumberbatch's Holmes
Even though they're playing the same character, RDJ has much respect.
"Very impressed with Benedict. Such smart writing on that show. I have screen envy."
5. His Surprising Favorite Superhero
And he has a sweet reason for the choice.
"My 2-year-old is fond of Hawkeye, so just for today, he'll top my list."
6. Clearing His Mel Gibson-Iron Man Statement
Because, come on, did you really think he was serious?
"It was an offhanded remark to a journalist and friend. I have other projects in mind for Mel and I - sooner than later the Marvel Roster will be made public and all questions will be answered."
7. Why He Doesn't Have A Downy Sponsorship
Now I'm mad.
"No, and I'm incensed.
There's no E in their Downy, so if they wouldn't mind a rebrand with an additional vowel, I'd meet them halfway. "
8. Which TV Show He'd Do A Cameo On
Please be real.
"Agents of S.H.I.E.L.D."
"The Judge" opens in theaters on October 10.