Australian actor Ryan Corr, who plays Coby in Packed to the Rafters alongside Stewart's Jake and formerly Marais's Rachel, said that the engaged couple are "wonderful people" and have a very happy future together.
Corr told New Idea magazine: "You know, Jimmy is already talking about reading Shakespeare to [the baby] while it's still growing, so I think it's going to be a stunning marriage and a stunning parenthood.
"I mean, these are two of the most wonderful people you'll ever meet... and they'll make the most down-to-earth, wonderful parents."
Corr also said that he is "not very shocked" about the pregnancy news despite the fact that Marais recently launched her acting career in the US.
"Jessica's really great when it comes to career," he said, "but I know that she's really measured when it comes to starting a family."
Another former Packed to the Rafters cast member Allison Cratchley added: "This is fantastic news for Jessica. Family is obviously as important as work to her and that's great. Look at Cate Blanchett - able to combine great roles with enjoying her family life."
When they announced the pregnancy, Marais and Stewart said that they are very "excited" at the prospect of becoming parents.