I thought the movie was great but all the scenes with the Charlotte Rampling Character seemed to be quite strange - and we don't even need her for the plot. All the relation between her and Newman was weird. And the cheque scene at the office of the other Lawyer was very pushed - you don't need it to get another "twist"....
it was useful to allow the intimacy of corruption to be expressed. also, her character was quite ambivalent. she like/loved garvin, we see her tearing herself apart in the process of betraying him. in one crucial scene, the night before the trial, she actually braces him up. given that garvin was more-or-less betrayed by his wife he still holds a torch for, after his father-in-law rang him up on the false charges, the betrayal was even more of an injury. that's drama.
i thought her character added, in short, a significant depth to the drama. she did an exquisite job of communicating her interior conflict in a very subtle way.
What surprised me was that, unless I missed something, the betrayal had no effect on the case. She was sent in to be a spy but I didn't catch her passing along any inside information that helped the defense.
Also, it was very weird how the fellow attorney just randomly started going through her purse when he found the check.