Bill Bradford was pretty anxious to use Julie Gardner in trying to catch the dangerous gangster in this flick, especially after seeing what happened to the LAST guy that tried that. (not being too specific so not to spoil anything...)
Both Davis & Brent had made many movies prior to this, so gave good steady performances. Some twists or surprises would have made it even better...
You are right, of course, but prior to that Bill tried to get Julie to leave her boss' employ. Just being there was unsafe but she also kept Carston's books so that put her at greater risk. She wouldn't quit her job.
Bill did ask her to play an active role in the investigation but only after she told him she was the only one who could decipher the code used in his accounting books. Carston needed her so there's no way he'd harm her. His money was too important to him.
With Julie's help, they could wrap up the case on Carston that much sooner. Once he was behind bars, Bill wouldn't have to worry about her associating with or working with him. Still, he did attempt to keep her away from her. He arranged to have her 'arrested' and out of his reach. Unfortunately, her handlers weren't able to hang on to her.
Woman, man! That's the way it should be Tarzan. [Tarzan and his mate]