I think it's a terrible performance that is covered up by a clever script (Anita Loos). Hard as nails is easy for a non-actress - it takes real emotion out of the picture. Harlow was still a "non-pro" as Mae Clarke said of her. The things she had going for her were a lot of energy and she committed to what she was doing. The performance itself was relentlessly one-note. That one-note synched up with the writing, which made Red ruthless. However, it is clear that if the role had called for anything other than heartless drive, Harlow would have no idea how to deliver it. At times the movie came off ridiculous as there was nothing really seductive about her. It was just relentless aggression coupled with what we were meant to believe was a physical appeal no man could resist. This lack of apology made the character different and fun, but Harlow didn't have the ability to play it any other way. I think the wig was badly styled, and the color took a lot of her sex appeal away too. A lot of the performance feels like the director told her how to do it, and then she imitated it. The promo, the nod and "yes they do!" feels mechanical -no internal shift. And the transition from the slap to "do it again, I like it!" is effective because it's inhuman. Her limitations as an actress become part of the character. I think the movie's big flaw is Harlow is just not charming and seductive enough to make it believable that poor Chester Morris would be completely unable to resist her. Maybe if he were a "Human Bondage" type character, but he was just a guy. She wasn't all that.
It's a shame Clara Bow turned it down (they wanted too much of a future commitment from her). She'd have added the kind of humor that came from character, that sprang from an internal life, and she'd have been less mechanical and one note. She had genuine vibrance.