Designing cinematic compositions, which involves moving cameras through a scene, is essential yet challenging in filmmaking. Machinima filmmaking provides real-time virtual environments for exploring different compositions flexibly and efficiently. However, producing high-quality cinematic compositions in such environments still requires significant cinematography skills and creativity. This paper presents Cinemassist, a tool designed to support and enhance this creative process by generating a variety of cinematic composition proposals at both keyframe and scene levels, which users can incorporate into their workflows and achieve more creative results. At the crux of our system is a deep generative model trained on real movie data, which can generate plausible, diverse camera poses conditioned on 3D animations and additional input semantics. Our model enables an interactive cinematic composition design workflow where users can co-design with the model by being inspired by model-generated suggestions while having control over the generation process. Our user study and expert rating find Cinemassist can facilitate the design process for users of different backgrounds and enhance the design quality especially for users with animation expertise, demonstrating its potential as an invaluable tool in the context of digital filmmaking.