Grant plays the romantic lead as he always plays the romantic lead. As the dashing debonair man he always does, and you know what that works perfectly. Grant knows exactly what he is doing in the early romantic moments of the film and succeeds in his usual way. One could argue that he really is not doing all that much but really he is doing something that no other actor can really imitate, and that is the romantic Grant performance. Grant has very good chemistry with Irene Dunne which works well for the film. Their scenes together work, and their playful scenes with their adopted daughter also work well. This film is a Drama not a comedy so Grant shows more sides than just this side.
Later on in the film where it becomes more and more sad and such Grant performance becomes superior, and he shows some sides of himself that he usually keeps hidden. His scene where he pleads for the custody of his adopted daughter to the judge is perfect. He plays it emotionally true and he really could not be more effective. The final scenes where he is grieving over a tragic loss, Grant excels once again. His internal pain he expertly shown and Grant is just brilliant in these scene. He could have not been better and causes the conclusion of the film to have all the more strength. Although fairly standard Grant for one half, he is completely amazing in the other half.