One of the biggest streaming releases in the first few months of the 2026 TV schedule was Young Sherlock, starring Hero Fiennes Tiffin as the future world’s greatest detective and Dónal Finn as James Moriarty, a.k.a. Sherlock’s future nemesis. The show had a lot going for it, immediately breaking a record with the trailer and rocketing to become the #1 show on Prime Video after releasing in early March.
Now, nearly a month later, Prime has given the show another big push to show off the dynamic built by the two stars, who shared the story behind their chemistry read.
How Prime Video Is Promoting Young Sherlock
Don’t worry if you haven’t checked it out with an Amazon Prime subscription just yet for it to become a new Prime favorite – I’ll avoid any big plot spoilers, but you don’t need a breakdown of every plot point to see how the streamer dedicated a week to promoting Young Sherlock, well after it originally launched on March 4.
Article continues belowStarting on March 27 and extending until April 3 at 11:59 p.m. PT, the full first episode is available streaming for FREE on Prime Video’s YouTube Page. At the time of writing, with just hours to go before the end of the week of free access, it has accumulated nearly 1.6 million – yes, million – views and a comment section full of compliments.
While the show is of course named after Sherlock Holmes, the relationship between Sherlock and James is central to the entire plot as it unfolds, so it was essential that the right two actors were cast in the roles.
What The Stars Told Us About Their Chemistry Read
Speaking with CinemaBlend as a duo, Dónal Finn and Hero Fiennes Tiffin shared how that dynamic started right off the bat with their chemistry read. Tiffin landed the lead role of Sherlock first, after an initial Zoom audition (and previously working with executive producer Guy Ritchie on The Ministry of Ungentlemanly Warfare). Finn’s process started out with a self-tape, followed by meeting showrunner Matthew Parkhill and Tiffin. Finn shared:
I remember going in and I said, 'I would just love to do these scenes as kind of bold as they possibly can be, because I think that was really true to Moriarty's character,' but often in those circumstances, it's not just doing the scenes well. It's making sure that you have a connection or a rapport with Hero, and to speak to the man's character, he's an impossible person not to have a rapport with.
Dónal Finn could have given the best performance in the history of all the Moriartys across the many adaptations of Sir Arthur Conan Doyle’s works, but that wouldn’t be enough if he didn’t click with the actor already cast to play Sherlock. And according to Finn, it wasn’t hard to click with Tiffin. He went on:
So we got in there, and we started doing the scenes, and I was really enjoying it, but I understood the stakes of the thing, and naturally, I think you can have a bit of nerves, and Hero just turned to me at the end of the scene, and he was like, 'That was great.' He's like, 'I really enjoyed doing that with you.'
The actor, who’d previously worked on Prime Video’s The Wheel of Time before its cancellation in 2025, understandably had some nerves about auditioning to play the legendary James Moriarty in a project from Guy Ritchie opposite the person who’d already been cast as Sherlock. He went on to credit Tiffin with making him feel more at ease:
It spoke to his level of generosity that it totally took some nerves out of my system and allowed me to continue to kind of try and do my best work or whatever. And I remember leaving and just thinking, 'Whatever happens, I'm so grateful for that hour of acting that we've just done. And it will be what it will be.'
And as it turned out, “what it will be” was the co-lead role in a TV show that became the #1 series on Prime Video shortly after its release! Right now, just one day shy of a full month since the March 3 premiere, Young Sherlock is listed at #3 in the US. It’s impossible to say how much of that is due to the pilot streaming free on YouTube to pique the interest of potential viewers, but it certainly couldn’t have hurt.
Seated right next to Dónal Finn while he described his side of the chemistry read, Hero Fiennes Tiffin chimed in to heap his own share of praise on his costar. He shared:
I have to add that Dónal, and I'm not just trying to give him a compliment back since he's complimented me there, but I swear to God, I told everyone close to me as soon as I walked out from that day. I'd never been in that position, in a chemistry read of knowing that my job was safe, and obviously I didn't ever intentionally get complacent and relax at all, because you want to give everyone that, and everyone who came in was brilliant. But Dónal came in towards the end of the day, and within two and a half seconds, I remember thinking, 'This guy is going to make me look bad if I'm not careful.'
Talk about a compliment! Apparently, the pair really wasted no time in clicking, and what was comfortable between them during the audition translated to showrunner Matthew Parkhill and the casting team, because Dónal Finn officially became the Moriarty to Hero Fiennes Tiffin’s Sherlock. Tiffin went on:
He is a sensational actor, and people are going to see it on screen. You don't have to take my word for it. Watch the show, and you'll see he is. He's incredible. I was fairly confident he was going to be the one playing Moriarty from the minute he sat down. And I couldn't imagine anyone [else]. Everyone was great, but I can't imagine anyone else in the role.
If you still haven’t checked out Young Sherlock or at least been tempted to see what these two are like on screen after a sensational chemistry read, you can get a glimpse of their dynamic in the record-breaking preview. Take a look at the trailer, which earned a whopping 223 million views over its first week online back in February, below:
All eight episodes of Young Sherlock are available streaming with a Prime Video subscription now, and there are still some hours left before the opportunity to stream the premiere for free expires on YouTube. The show has not yet been renewed for Season 2, but hopefully Prime giving the show a promotional push by releasing the premiere at no cost is a sign that there's still interest in continuing the saga of Sherlock, Moriarty, and the rest.
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