In a new interview with Digital Spy, Tom provides some season 3 news which sort of containdicates (or adds a little confusion to) what was implied in the past. We knew that there were reshoots for season 3 in March, as David S. Goyer kindly shared photos of the cast from the set. When Starz announced season 3 would be the last, in July, Chris Albrecht told Deadline that the decision was made after season 3 was already shot.
The decision to end Da Vinci’s Demons was made after Season 3 was shot, so the producers didn’t necessarily plan the season finale as a series finale. Still, Albrecht promises “a very satisfying conclusion” and a satisfying way for fans to get to it by getting access to the entire final season on-demand the day of the premiere.
In the Digital Spy interview, Tom reveals more about the season 3 closure, but as the timings in the article don't fit, there must have been more season 3 filming after March. Unless they knew before March that it was the end. I am only guessing, I really don't know anything, but Starz were close to announcing a season 3 air date in February. Read it in full on Digital Spy.
"John was fighting for character the entire time," he said. "It just made for far more interesting, grounded scripts than we've ever had before. So we're working off our best scripts easily this season. "We were aware of the show's flaws, and people who ran the show were aware of what hadn't quite worked. But because you're so entrenched in it, it's very difficult to see how to fix it from within. So we owe a great deal to John for this season's success."
Once US cable network Starz announced that there would be no fourth season of Da Vinci's Demons, Riley and his co-stars returned to set for reshoots - designed to give the series a more definitive ending.
"It was always the most ambitious finale we've ever done," he said. "It goes out with a bang - it's huge and expensive, so it would've always been great. But now we've just added the icing on the cake, to make it feel like more of an end."
New scenes slotted in throughout the 10 episodes will help to bring the stories of Leonardo and friends to a satisfactory close, Riley explained.
"It'll be as minimally open-ended as we can make it," he said. "It was time to end it and we've done it in a way we're really satisfied with. We manage to close off most of the storylines, and give all the lead characters a satisfying ending, so people will be pleased."