Seyfried met her fiancé, Thomas Sadoski, while filming the movie, and MacLaine couldn't help but gush about the on-set romance.
"Absolutely the best! I watched their love affair, and I watched the baby almost being conceived," MacLaine, 82, quipped. "To watch the two of them together... they're truly in love."
ET also spoke with Sadoski at the premiere, and naturally, he was just as enamored with his bride-to-be as MacLaine.
"I'm blown away by her every day," he admitted.
Though the pair didn't reveal the due date or their baby's gender or name, the 40-year-old actor did say that he's getting ready for fatherhood with a whole lot of preparation -- and by letting his lady lead the way.
"She does look about ready to pop. It's a pretty amazing thing. I just get to sort of hang back and watch it all happen," he added. "That's the great joy for me, trying to learn as much as I can -- try to stay out of the way and not screw anything up too much."
Seyfried and Sadoski announced their engagement in September and the pregnancy two months later.
For her latest red carpet turn, Seyfried opted for a black Chloe dress with mint green-and-white floral embroidery, which she paired with gold drop earrings and a bold red lip. To see even more gorgeous pregnancy styles from your fave celebs, click through the gallery below.