This is a comedy thriller about a serial killer, so it's a very black comedy. Rod Steiger is a failed actor under the influence of his dead mother, once a darling of the Broadway stage. He ritually kills older women while in character roles- an Irish priest, a gay hairdresser, a tough cop - while he plays mind games with the investigator on the case (George Segal).
Lee Remick is a witness who has an affair with the detective. The romance is enjoyable, and it works as a thriller too. The comedy mostly centres around Steiger playing various archetypes. As the Irish clergyman, he mimics Barry Fitzgerald. As a French gourmet, he's Maurice Chevalier. When he's the policemen he impersonates George Segal...
There's not much for the other actors to do other than let him get on with it. Segal is sympathetic as the hangdog investigator who keeps getting taken off the case. Eileen Heckart makes a comic impression as the most stereotypical Jewish mother ever. Remick's role is mostly decorative, though she is a fine comedy actor.
There's a strong visual impression of Manhattan with well chosen locations. The film has the potential to be upsetting, because the murders are quite graphic for the time, especially when set against Steiger's scenery shredding. The ending is a little predictable. But it's an engaging, imaginative police drama.