The couple are expecting their first child together and while the 'Newsroom' star admitted he is ''more terrified'' than he's ever been at the prospect of parenthood, his worries have been eased by how capable his fiancée is and he couldn't be more ''proud'' of her.
He told Us Weekly magazine: ''I am more proud of Amanda than I've ever been of anybody in my entire life because she's been an incredible mom already, and we haven't even gotten to go time yet.
''She's just so thoughtful, careful, loving, caring, giving, kind, generous, fun, funny. You can see it all going in there.
''And that level of attention that is being paid already, with all of that focus and energy and that kindness and goodness of spirit and heart, it's an amazing thing to witness.''
The 40-year-old actor first met Amanda when they starred together in 2015 play 'The Way We Get By' and their romantic relationship began when they filmed movie 'The Last Word' last year.
And Thomas admitted he had long been an admirer of Amanda's work before they collaborated together, and starring alongside the 31-year-old beauty only made him more impressed by her.
He said: ''First and foremost, she's one of the artists in this business that I have a tremendous amount -- and have had a tremendous amount -- of respect for for a very long time.
''[Working with her] did nothing but sort of reinforce my belief that she is an incredibly courageous, incredibly dedicated and incredibly soulful artist.''
Meanwhile, Amanda recently admitted she is ''ready'' to give birth.
The heavily-pregnant star said: ''It's good [but] I'm ready... I'm ready... I'm ready to meet the kid.''
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