"It is amazing and Jess is doing a phenomenal job," Leech told ET's Nischelle Turner at the film's premiere at Cineworld Leicester Square in London on Monday. "She's just taking in her stride. So it is incredible and you're sitting there going, 'Look at how your body is changing!' And then sometimes you're [like], 'Isn't it amazing?!' and then sometimes you go, 'What!' And I mean [that] in a good way."
Leech and Herman -- who showed off her baby bump in a stunning gold sequin gown -- were all smiles on the red carpet. When asked what lesson he's learned so far from his wife's pregnancy, he said, "Really listen to the question, and be careful how you answer."
"That would be my advice. Really listen," he said with a laugh. "If she says, 'Do I look fat in this, that's not what she's actually asking."
The couple, who got engaged in February 2018, tied the knot on Jan. 5, 2019, and has had an incredible year as newlyweds. While they await the arrival of their baby, Leech is already planning on having more.
"I want four [kids]," Leech told ET, adding, "And Jess' response to that probably can't be broadcast on national television. So she's like, 'Yeah, good luck. See you there.'…Fingers crossed everything goes brilliantly with this and we'll see then."
Meanwhile, as fans get excited about the long-awaitedDownton Abbey film to hit theaters, Leech hopes that they love the story.
"With this, we know there are people that love the show," he explained. "We just hope they love this story. I think it's a brilliant movie in that it takes all those characters that people love and puts them in a situation you've never seen them in and either hilarity or drama or tears ensue."
Downton Abbey arrives in theaters on Sept. 20. See more in the video below.